science/libkml: Fix build with Clang 16

Clang 16 uses c++17 by default.

science/libkml/Makefile

@@ -19,6 +19,7 @@ USES=		cmake compiler:c++11-lang minizip pathfix
 CMAKE_OFF=	BUILD_TESTING WITH_JAVA WITH_PYTHON WITH_SWIG
 CMAKE_ON=	BUILD_SHARED_LIBS
 LDFLAGS+=	-lz
+USE_CXXSTD=	c++11
 USE_LDCONFIG=	yes
 
 USE_GITHUB=	yes