Avoid duplicate definition of feature macro

Feature macros need to be defined before #including any headers,
preferably through the build system, but this is good enough.

Fixes a compile error on my fork's Travis CI.

src/core.c

@@ -88,6 +88,11 @@
 
 #include "raylib.h"
 
+#if (defined(__linux__) || defined(PLATFORM_WEB)) && _POSIX_S_SOURCE < 199309L
+    #undef _POSIX_C_SOURCE
+    #define _POSIX_C_SOURCE 199309L // Required for CLOCK_MONOTONIC if compiled with c99 without gnu ext.
+#endif
+
 #include "rlgl.h"           // raylib OpenGL abstraction layer to OpenGL 1.1, 3.3+ or ES2
 #include "utils.h"          // Required for: fopen() Android mapping
 
@@ -110,10 +115,6 @@
     #include "external/rgif.h"   // Support GIF recording
 #endif
 
-#if defined(__linux__) || defined(PLATFORM_WEB)
-    #define _POSIX_C_SOURCE 199309L // Required for CLOCK_MONOTONIC if compiled with c99 without gnu ext.
-#endif
-
 #include <stdio.h>          // Standard input / output lib
 #include <stdlib.h>         // Required for: malloc(), free(), rand(), atexit()
 #include <stdint.h>         // Required for: typedef unsigned long long int uint64_t, used by hi-res timer

src/gestures.h

@@ -148,7 +148,10 @@ float GetGesturePinchAngle(void);                       // Get gesture pinch ang
     int __stdcall QueryPerformanceCounter(unsigned long long int *lpPerformanceCount);
     int __stdcall QueryPerformanceFrequency(unsigned long long int *lpFrequency);
 #elif defined(__linux__)
-    #define _POSIX_C_SOURCE 199309L // Required for CLOCK_MONOTONIC if compiled with c99 without gnu ext.
+    #if _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 <sys/time.h>           // Required for: timespec
     #include <time.h>               // Required for: clock_gettime()
 #endif

src/physac.h

@@ -235,6 +235,22 @@ PHYSACDEF void ClosePhysics(void);
 
 #if defined(PHYSAC_IMPLEMENTATION)
 
+#if defined(_WIN32)
+    // Functions required to query time on Windows
+    int __stdcall QueryPerformanceCounter(unsigned long long int *lpPerformanceCount);
+    int __stdcall QueryPerformanceFrequency(unsigned long long int *lpFrequency);
+#elif (defined(__linux__) || defined(__APPLE__) || defined(PLATFORM_WEB))
+    #if _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
+    //#define _DEFAULT_SOURCE         // Enables BSD function definitions and C99 POSIX compliance
+    #include <sys/time.h>           // Required for: timespec
+    #include <time.h>               // Required for: clock_gettime()
+    #include <stdint.h>
+#endif
+
+
 #if !defined(PHYSAC_NO_THREADS)
     #include <pthread.h>            // Required for: pthread_t, pthread_create()
 #endif
@@ -248,18 +264,6 @@ PHYSACDEF void ClosePhysics(void);
 
 #include "raymath.h"                // Required for: Vector2Add(), Vector2Subtract()
 
-#if defined(_WIN32)
-    // Functions required to query time on Windows
-    int __stdcall QueryPerformanceCounter(unsigned long long int *lpPerformanceCount);
-    int __stdcall QueryPerformanceFrequency(unsigned long long int *lpFrequency);
-#elif defined(__linux__) || defined(__APPLE__) || defined(PLATFORM_WEB)
-    #define _POSIX_C_SOURCE 199309L // Required for CLOCK_MONOTONIC if compiled with c99 without gnu ext.
-    //#define _DEFAULT_SOURCE         // Enables BSD function definitions and C99 POSIX compliance
-    #include <sys/time.h>           // Required for: timespec
-    #include <time.h>               // Required for: clock_gettime()
-    #include <stdint.h>
-#endif
-
 //----------------------------------------------------------------------------------
 // Defines and Macros
 //----------------------------------------------------------------------------------