Suppress -Wempty-body warnings in GCC 6.x and later.

libc++ in LLVM 11 uses an empty else clause in
include/c++/v1/__thread_support which triggers this warning.

Reviewed by:	dim, emaste
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D26257

share/mk/bsd.sys.mk

@@ -150,7 +150,8 @@ CWARNFLAGS+=	-Wno-error=address			\
 
 # GCC 6.1.0
 .if ${COMPILER_VERSION} >= 60100
-CWARNFLAGS+=	-Wno-error=maybe-uninitialized		\
+CWARNFLAGS+=	-Wno-error=empty-body			\
+		-Wno-error=maybe-uninitialized		\
 		-Wno-error=nonnull-compare		\
 		-Wno-error=redundant-decls		\
 		-Wno-error=shift-negative-value		\