configure: use the nghttp2 path correctly with pkg-config

When --with-nghttp2 was used (without a given path), the
PKG_CONFIG_LIBDIR varialbe could get clobbered and ruin a proper
detection of the library.

Reported-by: Dilyan Palauzov
Bug: http://curl.haxx.se/mail/lib-2014-04/0159.html