strndupa.h: #error if building with non-GCC on non-GNU systems

At the moment only GCC is supported on non-GNU systems, unfortunately.
GNU systems usually have GLIBC, or another library with strndupa().
This strndupa() implementation requires GCC for the builtin alloca API.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

strndupa.h

@@ -49,6 +49,8 @@
 	(char *)memcpy(dst_, src, len_);		\
 }))
 
+#else  /* If not GCC, e.g. Clang */
++#error strndupa() may use an unsupported GNU C API, please forward any fix to maintainer, cheers!
 #endif /* __GNUC__ */
 #endif /* !HAVE_STRNDUPA */