Adding support for generic build environments

[LiamBindle]

Hi everyone,

This is a feature request for generalizing ESMA_cmake to work in generic build environments. Specifically, it's to get esma.cmake to work on systems that don't have "BASEDIR" so that external projects like GCHP can use the CMakeLists in repos like GEOS-ESM/MAPL and GEOS-ESM/FVdycoreCubed_GridComp with only minor changes.

For anyone that is not familiar with esma.cmake, it essentially does two things:

1. It defines macros that are used in CMakeLists in ESMA projects.
2. It sets variables for the paths to ESMA's dependencies on the host system (e.g. INC_ESMF for ESMF's include directories). This is actually done in FindBaselibs.cmake which is called in esma.cmake.

Currently, esma.cmake almost works in generic build environments, but the approach used to do (2) is specific to build environments that have BASEDIR. This is because FindBaselibs.cmake hardcodes paths relative to ${BASEDIR}.

See PR #29 for my proposed fix.

Let me know what you think.

Liam