Support non-vcpkg dependencies in source builds

[sjperkins]

When building from source, the vcpkg compiled dependencies aren't packaged into the distribution:

• Compilation failing on RH8 with GCC 11.3.0 #29 (comment)

For source builds, its probably reasonable to expect the C++ dependencies to be compiled and installed in the default location. i.e. /usr and for the build to depend on these, rather than the vcpkg compiled dependencies.

This would require modifying the cmake to search for the appropriate dependencies -- not sure how easy this would be with casacore.

Regardless, source builds are low priority as wheels package and link the compiled dependencies correctly -- this should be the preferred method of installation.