What is xmmintrin.h, and why aren't ports finding it?

[Chris H]

Greetings,
 Working on a recent 11-CURRENT install
(11-CURRENT #1 amd64 r274134 Nov 5 12:56:14 PST 2014)
svn info /usr/ports Revision: 372176

Given the above, and the fact that I have installed lang/gcc-48.
Is there any reason that any port wanting to include xmmintrin.h
fails to find it? Even though dmesg && messages reflects the fact
that gcc48 is included within my $PATH?

This is causing me no end of grief, and try as I might, I
can't seem to get past this problem.

Thank you for all your time, and consideration.

--Chris