Building Pegasus thirdparty libraries costs long time, it would be meaningful
to reuse a built thirdparty directory when build Pegasus source code in different
directories.
This patch introduces an environment variable PEGASUS_THIRDPARTY_ROOT to indicate
the thirdparty directory, if it has been built, it can be skipt to save time and
disk space.
Building Pegasus thirdparty libraries costs long time, it would be meaningful to reuse a built thirdparty directory when build Pegasus source code in different directories.
This patch introduces an environment variable
PEGASUS_THIRDPARTY_ROOT
to indicate the thirdparty directory, if it has been built, it can be skipt to save time and disk space.