Move `build_config.rb` -> `build_config/default.rb`; ref #5098

In addition, update the documents referring `build_config.rb` which is
no longer used. The new `build_config.rb` describes the new configuration
structure in the comment.

2 files changed, 30 insertions, 28 deletions

diff --git a/doc/guides/compile.md b/doc/guides/compile.md
index ad57970e3..ab774a633 100644
--- a/doc/guides/compile.md
+++ b/doc/guides/compile.md
@@ -24,24 +24,26 @@ Optional:
 
 ## Build
 
-Inside of the root directory of the mruby source a file exists
-called *build_config.rb*. This file contains the build configuration
-of mruby and looks like this for example:
+To compile `mruby` with the default build configuration, just invoke `rake`
+inside of the mruby source root. To generate and execute the test tools call
+`rake test`. To clean all build files call `rake clean`. To see full command
+line on build, call `rake -v`.
+
+You can specify your owne configration file by the `MRUBY_CONFIG` environment
+variable (you can use `CONFIG` for shorthand for `MRUBY_CONFIG`). If the path
+doesn't exist, *build_config/${MRUBY_CONFIG}.rb* is used. The default
+configuration is defined in the `build_config/default_config.rb` file.
+
+Those build configuration files contain the build configurationof mruby, for
+example:
+
 ```ruby
 MRuby::Build.new do |conf|
   toolchain :gcc
 end
 ```
 
-All tools necessary to compile mruby can be set or modified here. In case
-you want to maintain an additional *build_config.rb* you can define a
-customized path using the *$MRUBY_CONFIG* environment variable. If the
-path doesn't exist, *build_config/${MRUBY_CONFIG}.rb* is used.
-
-To compile just call `rake` inside of the mruby source root. To
-generate and execute the test tools call `rake test`. To clean
-all build files call `rake clean`. To see full command line on
-build, call `rake -v`.
+All tools necessary to compile mruby can be set or modified here.
 
 ## Build Configuration
 
@@ -306,11 +308,11 @@ When debugging mode is enabled
 
 ## Cross-Compilation
 
-mruby can also be cross-compiled from one platform to another. To
-achieve this the *build_config.rb* needs to contain an instance of
-`MRuby::CrossBuild`. This instance defines the compilation
-tools and flags for the target platform. An example could look
-like this:
+mruby can also be cross-compiled from one platform to another. To achieve
+cross-compilation, the build configuration needs to contain an instance of
+`MRuby::CrossBuild`. This instance defines the compilation tools and flags
+for the target platform. An example could look like this:
+
 ```ruby
 MRuby::CrossBuild.new('32bit') do |conf|
   toolchain :gcc
@@ -320,8 +322,9 @@ MRuby::CrossBuild.new('32bit') do |conf|
 end
 ```
 
-All configuration options of `MRuby::Build` can also be used
-in `MRuby::CrossBuild`.
+All configuration options of `MRuby::Build` can also be used in
+`MRuby::CrossBuild`. You can find examples under the *build_config*
+directory.
 
 ### Mrbtest in Cross-Compilation
 
diff --git a/doc/guides/mrbgems.md b/doc/guides/mrbgems.md
index ffbf17f26..4246587ce 100644
--- a/doc/guides/mrbgems.md
+++ b/doc/guides/mrbgems.md
@@ -5,16 +5,15 @@ standardised way into mruby.
 
 ## Usage
 
-By default mrbgems is currently deactivated. As soon as you add a GEM to your
-build configuration (i.e. *build_config.rb*), mrbgems will be activated and the
-extension integrated.
+You have to activate mrbgems explicitly in your build configuration.  To add
+a GEM, add the following line to your build configuration file, for example:
 
-To add a GEM into the *build_config.rb* add the following line for example:
 ```ruby
 conf.gem '/path/to/your/gem/dir'
 ```
 
 You can also use a relative path which would be relative from the mruby root:
+
 ```ruby
 conf.gem 'examples/mrbgems/ruby_extension_example'
 ```
@@ -54,9 +53,9 @@ NOTE: `:bitbucket` option supports only git. Hg is unsupported in this version.
 
 There are instances when you wish to add a collection of mrbgems into mruby at
 once, or be able to substitute mrbgems based on configuration, without having to
-add each gem to the *build_config.rb* file.  A packaged collection of mrbgems
+add each gem to your build configration file.  A packaged collection of mrbgems
 is called a GemBox.  A GemBox is a file that contains a list of mrbgems to load
-into mruby, in the same format as if you were adding them to *build_config.rb*
+into mruby, in the same format as if you were adding them to the build config
 via `config.gem`, but wrapped in an `MRuby::GemBox` object.  GemBoxes are
 loaded into mruby via `config.gembox 'boxname'`.
 
@@ -73,7 +72,7 @@ must be saved with a *.gembox* extension inside the *mrbgems* directory to to be
 picked up by mruby.
 
 To use this example GemBox, we save it as `custom.gembox` inside the *mrbgems*
-directory in mruby, and add the following to our *build_config.rb* file inside
+directory in mruby, and add the following to your build configration file inside
 the build block:
 ```ruby
 conf.gembox 'custom'
@@ -184,12 +183,12 @@ Version requirement supports following operators:
 
 When more than one version requirements is passed, the dependency must satisfy all of it.
 
-You can have default gem to use as dependency when it's not defined in *build_config.rb*.
+You can have default gem to use as dependency when it's not defined in your build configuration.
 When the last argument of `add_dependency` call is `Hash`, it will be treated as default gem information.
 Its format is same as argument of method `MRuby::Build#gem`, expect that it can't be treated as path gem location.
 
 When a special version of dependency is required,
-use `MRuby::Build#gem` in *build_config.rb* to override default gem.
+use `MRuby::Build#gem` in the build configuration to override default gem.
 
 If you have conflicting GEMs use the following method:
 * `spec.add_conflict(gem, *requirements)`