Don't disable an unsupported compiler warning

Conditionalize the pragma that temporarily disables
-Wunused-const-variable.  Some versions of gcc don't support it.  We
need to do this because of an apparent bug in some libzstd headers.
Fixes bug 26785; bugfix on 0.3.2.11.

src/common/compress_zstd.c

@@ -19,10 +19,14 @@
 #include "compress_zstd.h"
 
 #ifdef HAVE_ZSTD
+#ifdef HAVE_CFLAG_WUNUSED_CONST_VARIABLE
 DISABLE_GCC_WARNING(unused-const-variable)
+#endif
 #include <zstd.h>
+#ifdef HAVE_CFLAG_WUNUSED_CONST_VARIABLE
 ENABLE_GCC_WARNING(unused-const-variable)
 #endif
+#endif
 
 /** Total number of bytes allocated for Zstandard state. */
 static atomic_counter_t total_zstd_allocation;