--minlinuxversion, when osxVersion >= (10, 5), etc

[timotheecour]

proposal

• add a way to query OS version (for OSX, ubuntu , linux kernel, windows version, gcc/clang version etc) at CT, as a tuple, which can then be used for conditional compilation
• add a flag to set this version for use in cross-compilation, analog to -mmacosx-version-min=version refs https://stackoverflow.com/questions/25352389/what-is-the-difference-between-macosx-deployment-target-and-mmacosx-version-min

example

when osxVersion >= (10, 5): # use copyfile API
else: # use fallback version

refs nim-lang#16883 (comment)

links

similar approach to proposal 5 from nim-lang/RFCs#205