Support multiple compilers side-by-side in build

[LKedward]

Currently compiled binaries are placed in build/debug/ and build/release.
Would it be possible to make this build path dependent on compiler (and maybe architecture)?

e.g. build/gfortran-9.2.0-x86_64_debug/

This reason for this being that I like to build and test my projects on multiple compilers during development; separating the binary output paths allows incremental builds using multiple compilers and allows tests on each compiler to run simultaneously and without clean and rebuild.
This may be considered an 'ugly' directory structure, however since fpm handles all compiler/linker paths and offers the fpm run command, this has no effect on end-user experience.

The workflow for using a non-default compiler could then be along the lines of:

$> export FC=ifort
$> fpm run

or

$> FC=ifort fpm run

or

$> fpm run --compiler=ifort

[everythingfunctional]

Yeah, this shouldn't be too difficult. Good idea.

[awvwgk]

This has been resolved with #255 in Fortran fpm and #220 in the Haskell version.