Bug 1295811 - Check COMPILE_ENVIRONMENT instead of SKIP_COMPILER_CHECKS; r=glandium

Reviewed over pastebin+irc, like a boss.

old-configure.in

@@ -2056,10 +2056,12 @@ fi
 
 AC_HAVE_FUNCS(localeconv)
 
-MOZ_CHECK_ALLOCATOR
-
 fi # ! SKIP_COMPILER_CHECKS
 
+if test -n "${COMPILE_ENVIRONMENT}"; then
+  MOZ_CHECK_ALLOCATOR
+fi
+
 TARGET_XPCOM_ABI=
 if test -n "${CPU_ARCH}" -a -n "${TARGET_COMPILER_ABI}"; then
     TARGET_XPCOM_ABI="${CPU_ARCH}-${TARGET_COMPILER_ABI}"