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