Add back $SAGE_LOCAL/bin to PATH in build/install
Trac #15687: Overzealous compiler check when building docs

The compiler/linker check from #15580 broke because `$SAGE_LOCAL/bin`
was removed from the `$PATH`: http://build.sagemath.org/sage/builders/%2
0%20slow%20Skynet%20mark%20%28SunOS%205.10-32%29%20full/builds/0/steps/c
ompile_2/logs/stdio

URL: http://trac.sagemath.org/15687
Reported by: vbraun
Ticket author(s): Jeroen Demeyer
Reviewer(s): Volker Braun