[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 <jaylcypher@github.com>

src/rcore.c

@@ -82,6 +82,19 @@
 *
 **********************************************************************************************/
 
+//----------------------------------------------------------------------------------
+// Feature Test Macros required for this module
+//----------------------------------------------------------------------------------
+#if (defined(__linux__) || defined(PLATFORM_WEB)) && (_XOPEN_SOURCE < 500)
+    #undef _XOPEN_SOURCE
+    #define _XOPEN_SOURCE 500 // Required for: readlink if compiled with c99 without gnu ext.
+#endif
+
+#if (defined(__linux__) || defined(PLATFORM_WEB)) && (_POSIX_C_SOURCE < 199309L)
+    #undef _POSIX_C_SOURCE
+    #define _POSIX_C_SOURCE 199309L // Required for: CLOCK_MONOTONIC if compiled with c99 without gnu ext.
+#endif
+
 #include "raylib.h"                 // Declares module functions
 
 // Check if config flags have been externally provided on compilation line
@@ -235,11 +248,6 @@ __declspec(dllimport) int __stdcall WideCharToMultiByte(unsigned int cp, unsigne
 #define FLAG_TOGGLE(n, f) ((n) ^= (f))
 #define FLAG_CHECK(n, f) ((n) & (f))
 
-#if (defined(__linux__) || defined(PLATFORM_WEB)) && (_POSIX_C_SOURCE < 199309L)
-    #undef _POSIX_C_SOURCE
-    #define _POSIX_C_SOURCE 199309L // Required for: CLOCK_MONOTONIC if compiled with c99 without gnu ext.
-#endif
-
 //----------------------------------------------------------------------------------
 // Types and Structures Definition
 //----------------------------------------------------------------------------------

src/rglfw.c

@@ -37,6 +37,18 @@
 // _GLFW_OSMESA     to use the OSMesa API (headless and non-interactive)
 // _GLFW_MIR        experimental, not supported at this moment
 
+//----------------------------------------------------------------------------------
+// Feature Test Macros required for this module
+//----------------------------------------------------------------------------------
+#if (defined(__linux__) || defined(PLATFORM_WEB)) && (_POSIX_C_SOURCE < 199309L)
+    #undef _POSIX_C_SOURCE
+    #define _POSIX_C_SOURCE 199309L // Required for: CLOCK_MONOTONIC if compiled with c99 without gnu ext.
+#endif
+#if (defined(__linux__) || defined(PLATFORM_WEB)) && !defined(_GNU_SOURCE)
+    #undef _GNU_SOURCE
+    #define _GNU_SOURCE // Required for: ppoll if compiled with c99 without gnu ext.
+#endif
+
 #if defined(_WIN32) || defined(__CYGWIN__)
     #define _GLFW_WIN32
 #endif