Fix error in build with gcc10
--- a/configure.ac
+++ b/configure.ac
@@ -44,7 +44,7 @@
 	fi
 fi
 
-CFLAGS="$CFLAGS $OPENMP_CFLAGS"
+CFLAGS="-Werror=unknown-pragmas -Werror=unused-label -fcommon $CFLAGS $OPENMP_CFLAGS"
 CXXFLAGS="$CFLAGS"
 
 dnl Set PACKAGE_SOURCE_DIR in config.h.
