Change internal flag organization in CMake build to not be global (#2053)

Change internal flag organization in CMake build to not be global (#2053)

TYPE: enhancement

KEYWORDS: cmake, flags, compilation

SOURCE: internal

DESCRIPTION OF CHANGES:
Problem:
The current iteration of the cmake build places all configuration flags
in the global properties of the project. While this works when just
building WRF, integration with other projects' cmake builds if placed
under WRF pollutes their respective build flags.

Solution:
Adjust the layout of the flags to instead carry them in a variable, and
prefer using `target_*` calls for flag usage.

LIST OF MODIFIED FILES:
M       CMakeLists.txt
M       chem/CMakeLists.txt
M       external/CMakeLists.txt
M       external/io_adios2/CMakeLists.txt
M       external/io_netcdf/CMakeLists.txt
M       external/io_netcdfpar/CMakeLists.txt
M       external/io_pnetcdf/CMakeLists.txt
M       frame/CMakeLists.txt
M       main/CMakeLists.txt
M       phys/CMakeLists.txt
M       tools/CMakeLists.txt

RELEASE NOTE:
Change internal flag organization in CMake build to not be global