Use __GLIBC_PREREQ only when __GLIBC__ is defined

The way __GLIBC_PREREQ() is currently used means that it's evaluated
even if __GLIBC__ is not defined. But obviously, __GLIBC_PREREQ will
not exist if __GLIBC__ is not defined, causing build failures on C
libraries not defining __GLIBC__ such as the musl C library.

Patch originally taken from:
https://github.com/voidlinux/void-packages/blob/master/srcpkgs/numactl/patches/musl.patch

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: improve patch description.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

--- a/syscall.c.orig	2014-10-20 16:12:53.000000000 +0200
+++ b/syscall.c	2015-06-22 08:13:22.729034702 +0200
@@ -115,14 +115,16 @@
 
 #endif
 
-#if defined(__GLIBC__) && __GLIBC_PREREQ(2, 11)
+#if defined(__GLIBC__)
+# if __GLIBC_PREREQ(2,11)
 
 /* glibc 2.11 seems to have working 6 argument sycall. Use the
    glibc supplied syscall in this case.
    The version cut-off is rather arbitary and could be probably
    earlier. */
 
-#define syscall6 syscall
+#  define syscall6 syscall
+#endif
 #elif defined(__x86_64__)
 /* 6 argument calls on x86-64 are often buggy in both glibc and
    asm/unistd.h. Add a working version here. */