fix build error on ARM due to wrong glibc check

the test for glibc < 2 "succeeds" wrongly for any non-glibc C library,
and breaks the build on musl libc.
we must first test if __GLIBC__ is defined at all, before using it
unconditionally.
Signed-off-by: NJohn Spencer <maillist-qemu@barfooze.de>
Reviewed-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

user-exec.c

@@ -436,7 +436,7 @@ int cpu_signal_handler(int host_signum, void *pinfo,
     unsigned long pc;
     int is_write;
 
-#if (__GLIBC__ < 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ <= 3))
+#if defined(__GLIBC__) && (__GLIBC__ < 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ <= 3))
     pc = uc->uc_mcontext.gregs[R15];
 #else
     pc = uc->uc_mcontext.arm_pc;