build: fix configuring with prebuilt libraries

Fix configure_library() to produce correct LDFLAGS when configuring with
prebuilt 3rd-party libraries (libuv, openssl, etc) using `pkg-config' or
`--shared-{LIBRARY}-includes=xxx --shared-{LIBRARY}-libpath=xxx'.

PR-URL: https://github.com/nodejs/node/pull/3135


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>