Add user-defined-flags to fortran in toml

[connoraird]

A possible solution to issue 1096.

Description

I have added a new option/line under [fortran] for fpm.toml files such that we can now add

name = "some-project"

[fortran]
user-defined-flags = "<some-flag>"

These user-defined-flags are then directly passed through to the fortran compiler. Therefore, the compiler handles displaying errors with provided flags.

Testing

I have added this new fpm.toml option to some existing tests but I am not certain this is fully testing the new feature. Any input would be appreciated.

[JorgeG94]

One issue at the moment is that certain files that are passed at compile time should also be passed at link time, for example if we want to pass -fopenacc or -fopenmp. These two should act as "dependencies" that should be included both at compile and link time. This could get annoying