Implement option for using non-prefixed environment variables

[awvwgk]

Description

For integration in packaging systems like conda-build or spack or to seamlessly work with environment modules fpm could allow an option to use non-prefixed environment variables.

• --compiler, FPM_FC, FC to set Fortran compiler
• --c-compiler, FPM_CC, CC to set C compiler
• --archiver, FPM_AR, AR to set archiver
• --flag, FPM_FFLAGS, FFLAGS to set Fortran compiler flags
• --c-flag, FPM_CFLAGS, CFLAGS to set C compiler flags
• --link-flag, FPM_LDFLAGS, LDFLAGS to set linker flags

The non-prefixed environment variables will not be used by default to avoid picking up environment variables by accident, i.e. using the non-prefixed variables is always opt-in.

Possible Solution

Use command line flag --env or similar to activate usage of compiler variables. Always provide an option to show to show where a particular default originated from.

Additional Information

Follow-up from #549

[LKedward]

Comment from @urbanjost :

One other change that I was hoping would come out of the discussions was an option to set the variable prefix so that it could be a user-defined value, even a blank character, so you could set compiler-specific values or even not have a prefix (and so use FC, AR, ...). So I could set INTEL_FFLAGS and GFORTRAN_FFLAGS and ... and switch to different sets of them just by setting FPM_PRE. No one liked that? It would give you a back door to use the default names and if you are using environment variables would let you have compiler-specific sets of them.

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This use case of different compiler flags is a compelling reason to do this IMO.

[urbanjost]

I think the proposed PR #608 satisfied the original post without introducing some of the perils associated with using environment variables as an opt-in is required, and easily allows setting groups of variables for specific compilers. It also allows for user-defined names for groups of variables for the case where you want multiple choices for the same compiler. All new variable names are required to begin with FPM_ so they are easier to identify and list as well.