bsd.sys.mk: Add NO_WUNUSED_BUT_SET_VARIABLE helper variable.

This is already present in sys/conf/kern.mk and can be used to
selectively disable -Wunused-but-set-variable.

Reviewed by:	emaste
Differential Revision:	https://reviews.freebsd.org/D40663

share/mk/bsd.sys.mk

@@ -116,6 +116,9 @@ CWARNFLAGS.clang+=	-Wno-array-bounds
       ${COMPILER_TYPE} == "gcc")
 CWARNFLAGS+=		-Wno-misleading-indentation
 .endif # NO_WMISLEADING_INDENTATION
+.if ${COMPILER_VERSION} >= 130000
+NO_WUNUSED_BUT_SET_VARIABLE=	-Wno-unused-but-set-variable
+.endif
 .if ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 140000
 NO_WBITWISE_INSTEAD_OF_LOGICAL=	-Wno-bitwise-instead-of-logical
 .endif
@@ -124,6 +127,9 @@ NO_WARRAY_PARAMETER=	-Wno-array-parameter
 NO_WSTRICT_PROTOTYPES=	-Wno-strict-prototypes
 NO_WDEPRECATED_NON_PROTOTYPE=-Wno-deprecated-non-prototype
 .endif
+.if ${COMPILER_TYPE} == "gcc" && ${COMPILER_VERSION} >= 50200
+NO_WUNUSED_BUT_SET_VARIABLE=-Wno-unused-but-set-variable
+.endif
 .if ${COMPILER_TYPE} == "gcc" && ${COMPILER_VERSION} >= 100100
 NO_WZERO_LENGTH_BOUNDS=	-Wno-zero-length-bounds
 .endif