Fix configure issue that discarded provided CC definition.

Taken from upstream:

  https://github.com/madler/zlib/commit/05796d3d8d5546cf1b4dfe2cd72ab746afae505d

diff --git a/configure b/configure
index 52ff4a04e..3fa3e8618 100755
--- a/configure
+++ b/configure
@@ -174,7 +174,10 @@ if test -z "$CC"; then
   else
     cc=${CROSS_PREFIX}cc
   fi
+else
+  cc=${CC}
 fi
+
 cflags=${CFLAGS-"-O3"}
 # to force the asm version use: CFLAGS="-O3 -DASMV" ./configure
 case "$cc" in