Determine which bundled sources to distribute

The build_external_clib subcommand now automatically adds any sources
for bundled libraries to the distribution. It is no longer necessary to
name such sources explicitly in MANIFEST.in.
Run 'autoreconf -i' for any bundled libraries

For autotools-based packages, this generates the 'configure' script if
it does not already exist.
Optionally build against bundled cfitsio

cfitsio does not support building outside of the source directory, so
this commit also adds an optional 'supports_non_srcdir_builds' key to
build_info that instructs build_external_clib to copy the source into
the temporary build directory.
Add internally built pkgconfig directory to PKG_CONFIG_PATH

As a result, as external libraries are built, they get picked up by
subsequent libraries that depend upon them.