WITH_SYSTEM_COMPILER: Enable by default

This improves buildworld, toolchain, kernel-toolchain, and universe targets.
See r300354 or src.conf(5) for more details.

Approved by:	re (gjb)
Relnotes:	yes (r300354)
Sponsored by:	EMC / Isilon Storage Division

share/mk/src.opts.mk

@@ -158,6 +158,7 @@ __DEFAULT_YES_OPTIONS = \
     SOURCELESS_UCODE \
     SVNLITE \
     SYSCONS \
+    SYSTEM_COMPILER \
     TALK \
     TCP_WRAPPERS \
     TCSH \
@@ -189,7 +190,6 @@ __DEFAULT_NO_OPTIONS = \
     SHARED_TOOLCHAIN \
     SORT_THREADS \
     SVN \
-    SYSTEM_COMPILER \
 
 
 #