Fix build error in libc_compat when using musl libc

Checking for the existence of and using __GLIBC_PREREQ can't be done in the
same conditional.

Please refer: https://github.com/blender/blender/commit/7be7280c5710f7831789cdde140d010722be9068
--- a/intern/libc_compat/libc_compat.c
+++ b/intern/libc_compat/libc_compat.c
@@ -13,7 +13,8 @@
 #  include <features.h>
 #  include <math.h>

-#  if defined(__GLIBC_PREREQ) && __GLIBC_PREREQ(2, 31)
+#  if defined(__GLIBC_PREREQ)
+#    if __GLIBC_PREREQ(2, 31)

 double __exp_finite(double x);
 double __exp2_finite(double x);
@@ -112,5 +113,6 @@ float __powf_finite(float x, float y)
   return powf(x, y);
 }

-#  endif /* __GLIBC_PREREQ */
-#endif   /* __linux__ */
+#    endif /* __GLIBC_PREREQ(2, 31) */
+#  endif   /* __GLIBC_PREREQ */
+#endif     /* __linux__ */