Configure: added gcc5 to the list of known GCC versions.

auto/cc/gcc

diff --git a/auto/cc/gcc b/auto/cc/gcc
index 727f11e..c9101fe 100644
--- a/auto/cc/gcc
+++ b/auto/cc/gcc
@@ -153,7 +153,7 @@
 
 
 case "$NGX_GCC_VER" in
-    3.* | 4.* )
+    [3-5].*)
         # we have a lot of the unused function arguments
         CFLAGS="$CFLAGS -Wno-unused-parameter"
         # 4.2.1 shows the warning in wrong places