math/fenics-basix: Broken when std::concepts isn't available for clang

Reported by:	fallout

math/fenics-basix/Makefile

@@ -21,4 +21,10 @@ GH_PROJECT=	basix
 
 WRKSRC_SUBDIR=	cpp
 
-.include <bsd.port.mk>
+.include <bsd.port.pre.mk>
+
+.if ${CHOSEN_COMPILER_TYPE} == clang && !exists(/usr/include/c++/v1/concepts) # true only on FreeBSD_12
+BROKEN=		system is missing a required C++ feature std::concepts
+.endif
+
+.include <bsd.port.post.mk>