config BUGFIX include-time compiler attributes

Supported public compiler attributes should not
depend on the compiler used for compiling libyang
but on the compiler compiling the code including
libyang headers.

Refs #1739
diff --git a/src/config.h.in b/src/config.h.in
index e6bdd6a..de9c40c 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -29,7 +29,7 @@
 #define LY_ATOMIC_DEC_BARRIER(var) __sync_fetch_and_sub(&(var), 1)
 
 /** printf compiler attribute */
-#if (@CMAKE_C_COMPILER_ID@ == GNU) || (@CMAKE_C_COMPILER_ID@ == Clang)
+#ifdef __GNUC__
 # define _FORMAT_PRINTF(FORM, ARGS) __attribute__((format (printf, FORM, ARGS)))
 #else
 # define _FORMAT_PRINTF(FORM, ARGS)