When USES contains compiler:gcc-c++11-lib,
Mk/Uses/compiler.mk will force USE_GCC=yes,
instead of using USE_GCC?=yes.
This harms ports that set, for instance, USE_GCC=5+, by re-allowing GCC 4.9 (which is known to miscompile rawtherapee on FreeBSD 11.0-RELEASE amd64, while GCC 5.4 is fine).
change code to USE_GCC?=yes here:
USE_GCC= yes # <====-FIX-THIS-===============
CXXFLAGS+= -nostdinc++ -isystem /usr/include/c++/v1
re-set USE_GCC after .include <bsd.port.pre.mk>. (See, for instance, graphics/rawtherapee[-devel]).
I believe USE_GCC?=yes doesn't help, the workaround will still be needed.