Build: change libc compatibility code to only be used for new glibc

This way we can still use fast math functions.

intern/libc_compat/CMakeLists.txt

@@ -32,4 +32,6 @@ set(SRC
 set(LIB
 )
 
+add_c_flag(-ffast-math)
+
 blender_add_lib(bf_intern_libc_compat "${SRC}" "${INC}" "${INC_SYS}" "${LIB}")

intern/libc_compat/libc_compat.c

@@ -21,7 +21,11 @@
  * incompatible with the system libraries that Blender is built on. To solve
  * this we add a few -ffast-math symbols that can be missing. */
 
-#include <math.h>
+#ifdef __linux__
+#  include <features.h>
+#  include <math.h>
+
+#  if defined(__GLIBC_PREREQ) && __GLIBC_PREREQ(2, 31)
 
 double __exp_finite(double x);
 double __acos_finite(double x);
@@ -107,3 +111,6 @@ float __powf_finite(float x, float y)
 {
   return powf(x, y);
 }
+
+#  endif /* __GLIBC_PREREQ */
+#endif   /* __linux__ */