Fix usage of strerror_r

Only GNU libc has implementation of the GNU variant of strerror_r, so
also for __GLIBC__ to determine which strerror_r implemenation to use.

This fixes build with musl libc, which makes some, but not all, GNU
extensions available with _GNU_SOURCE.

src/futils.cpp

@@ -54,7 +54,7 @@
 #include <stdexcept>
 
 #ifdef EXV_HAVE_STRERROR_R
-#ifdef _GNU_SOURCE
+#if defined(__GLIBC__) && defined(_GNU_SOURCE)
 extern char *strerror_r(int errnum, char *buf, size_t n);
 #else
 extern int strerror_r(int errnum, char *buf, size_t n);
@@ -348,7 +348,7 @@ namespace Exiv2 {
         std::ostringstream os;
 #ifdef EXV_HAVE_STRERROR_R
         const size_t n = 1024;
-#ifdef _GNU_SOURCE
+#if defined(__GLIBC__) && defined(_GNU_SOURCE)
         char *buf = 0;
         char buf2[n];
         std::memset(buf2, 0x0, n);