Make FetchContent friendly

[scivision]

To modernize CMake packaging and allow easier/friendlier consumption of netCDF-C and netCDF-fortran from third-party projects, it would be good to make each project FetchContent-friendly by using modern CMake best practices.
This is especially important to allow netcdf-fortran to cleanly automatically build netcdf-c if it isn't found.

• use NC4F_SOURCE_DIR or NC4F_BINARY_DIR instead of CMAKE_SOURCE_DIR OR CMAKE_BINARY_DIR Implement CMake FetchContent-friendly modernizations #480
• don't use check_library_exists() tests that use try_compile() in netcdf-fortran when netcdf-c is presented via FetchContent
• ensure that all targets (currently a problem under docs/) have unique names between netcdf-c and netcdf-fortran
• only use enable_testing() if ENABLE_TESTS so that other projects don't run NetCDF internal tests unless requested