[CoreFoundation] Don't assume we can use __GLIBC_PREREQ on Linux.

We might be using musl instead, which doesn't have __GLIBC_PREREQ as
it isn't Glibc.

rdar://123381867

Author: al45tair

CoreFoundation/Base.subproj/ForSwiftFoundationOnly.h

@@ -69,6 +69,7 @@
 #include <features.h>
 #include <termios.h>
 
+#ifdef __GLIBC_PREREQ
 #if __GLIBC_PREREQ(2, 28) == 0
 // required for statx() system call, glibc >=2.28 wraps the kernel function
 #include <sys/syscall.h>
@@ -78,7 +79,7 @@
 #include <linux/fs.h>
 #define AT_STATX_SYNC_AS_STAT   0x0000  /* - Do whatever stat() does */
 #endif //__GLIBC_PREREQ(2. 28)
-
+#endif // defined(__GLIBC_PREREQ)
 #ifndef __NR_statx
 #include <sys/stat.h>
 #endif // not __NR_statx