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/LD_LIBRARY_PATH

* Add $BUILD_DIR/.heroku/vendor/include to
  C_INCLUDE_PATH/CPLUS_INCLUDE_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

bin/compile

@@ -79,11 +79,14 @@ ORIG_BUILD_DIR=$BUILD_DIR
 BUILD_DIR=$APP_DIR
 
 # Prepend proper path buildpack use.
-export PATH=$BUILD_DIR/.heroku/python/bin:$PATH
+export PATH=$BUILD_DIR/.heroku/python/bin:$BUILD_DIR/.heroku/vendor/bin:$PATH
 export PYTHONUNBUFFERED=1
 export LANG=en_US.UTF-8
-export LIBRARY_PATH=/app/.heroku/vendor/lib
-export LD_LIBRARY_PATH=/app/.heroku/vendor/lib
+export C_INCLUDE_PATH=/app/.heroku/vendor/include:$BUILD_DIR/.heroku/vendor/include
+export CPLUS_INCLUDE_PATH=/app/.heroku/vendor/include:$BUILD_DIR/.heroku/vendor/include
+export LIBRARY_PATH=/app/.heroku/vendor/lib:$BUILD_DIR/.heroku/vendor/lib
+export LD_LIBRARY_PATH=/app/.heroku/vendor/lib:$BUILD_DIR/.heroku/vendor/lib
+export PKG_CONFIG_PATH=/app/.heroku/vendor/lib/pkg-config:$BUILD_DIR/.heroku/vendor/lib/pkg-config
 
 # Switch to the repo's context.
 cd $BUILD_DIR