BuildPDAF: Add -fcommon to Fortran/C optimization flags

[jjokella]

Goal: remove multiple-definition errors for GCC compilers.

For more information, see HPSCTerrSys/pdaf#8

[kvrigor]

@jjokella Is this part of a larger effort related to fixing GCC issues on PDAF? Maybe it makes more sense for #78 and #79 to be a single PR.

[jjokella]

Hey @kvrigor, actually current build-checks indicate that these three PRs #77 #78 and #79 are enough to ensure compilation.

Maybe they are a little to scattered? But I think I would leave them now as is.

[kvrigor]

Maybe they are a little to scattered? But I think I would leave them now as is.

I think so too; I try to combine related commits so that TSMP2's git log is a nice series of feature sets instead of a bunch of micro-commits without a broader context. For now I'll approve the PRs.

[jjokella]

Will keep this in mind for future development