math/eclib: work-around to build with clang 16

Adding "CXXFLAGS+=-std=c++11" to fix the build on 14-CURRENT.

Without that, it fails with this error:

In file included from unimod.cc:24:
In file included from ./eclib/unimod.h:27:
In file included from ./eclib/interface.h:51:
./eclib/templates.h:64:12: error: no member named 'ptr_fun' in namespace 'std'
using std::ptr_fun;
      ~~~~~^
--- interface.lo ---
In file included from interface.cc:24:
In file included from ./eclib/interface.h:51:
./eclib/templates.h:64:12: error: no member named 'ptr_fun' in namespace 'std'
using std::ptr_fun;
      ~~~~~^
1 error generated.
*** [interface.lo] Error code 1

PR:		271047
Reported by:	pkg-fallout

math/eclib/Makefile

@@ -21,6 +21,7 @@ USES=		autoreconf compiler:c++11-lang libtool localbase
 
 OPTIONS_DEFINE=	DOCS
 
+CXXFLAGS+=	-std=c++11
 LIBS+=		"-lm"
 GNU_CONFIGURE=	yes
 CONFIGURE_ARGS=	--with-flint=${LOCALBASE} --with-boost=${LOCALBASE}