[rcore rglfw] Feature Test Macros before include (#3737)

Move/Add Feature Test Macros before any includes.
See: [GNU Feature Test Macros](https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html)
> You should define these macros by using ‘#define’ preprocessor directives at the top of your source code files. These directives must come before any #include of a system header file. It is best to make them the very first thing in the file, preceded only by comments.

Alternative changes to rcore would be to change _POSIX_C_SOURCE to
200809L, which removes the need to define _XOPEN_SOURCE >= 500.

These changes allow for compilation with -std=c* (such as -std=c99)
without adding -D macros to the build step.

Co-authored-by: JayLCypher <[email protected]>

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