Configure: changed default compiler from "gcc" to "cc".

This allows to automatically pick the preferred system compiler on
systems with multiple compilers.

Reviewed by:	mdounin, ru

auto/options

diff --git a/auto/options b/auto/options
index a75bead..150286d 100644
--- a/auto/options
+++ b/auto/options
@@ -15,7 +15,7 @@
 NGX_USER=
 NGX_GROUP=
 
-CC=${CC:-gcc}
+CC=${CC:-cc}
 CPP=
 NGX_OBJS=objs