Make vendored libraries available at compile time (fixes #57)

When building, BUILD_DIR is set to a temporary directory. Vendored
libraries (e.g., pylibmc) are downloaded and unpacked in
$BUILD_DIR/.heroku/vendor/lib, but this is not currently in
LIBRARY_PATH or LD_LIBRARY_PATH.

Fix this by doing the following:

* Add $BUILD_DIR/.heroku/vendor/lib to LIBRARY_PATH and
  LD_LIBRARY_PATH

* Add $BUILD_DIR/.heroku/vendor/bin to PATH, so vendored commands can
  be used at compile time (e.g., curl-config)

* Add $BUILD_DIR/.heroku/vendor/lib/pkg-config to PKG_CONFIG_PATH, so
  vendored packages can be found with pkg-config