build: fix --with-zlib when a path is specified #1292
Prior to this change if you attempted to configure curl using
Prior to this change if you attempted to configure curl using --wtih-zlib and specified a path the path would be ignored if you also had pkg-config installed on your system. This situation can easily arise when you are cross compiling. This change moves the test for detecting zlib settings via pkg-config only if OPT_ZLIB is not set.