Fix build on gcc-based architectures:

C++11 is required to use dlib, but the version of GCC you are using is too old and doesn't support C++11.  You need GCC 4.8 or newer.

Approved by:	portmgr (tier-2 blanket)

science/dlib-cpp/Makefile

@@ -12,7 +12,7 @@ COMMENT=	Machine learning framework written in C++
 LICENSE=	BSL
 LICENSE_FILE=	${WRKSRC}/dlib/LICENSE.txt
 
-USES=			cmake pkgconfig tar:bzip2
+USES=			cmake compiler:c++11-lang pkgconfig tar:bzip2
 USE_LDCONFIG=		yes
 
 OPTIONS_DEFINE=		X11 JPEG PNG GIF SQLITE BLASLAPACK PYTHON AVX