Switch back to USE_GCC=yes.

Work around this bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65009
in g++49 -Os optimzation by using -O0 optimization if we are compiling
with g++49.

Differential Revision: https://reviews.freebsd.org/D1607
Reviewed by: pfg
Approved by: mat (mentor)