CONFIG: provide separate default settings for clang/gcc (fixes #1566)

- the foamConfigurePaths script is quite simplistic and aggressive in
  what it changes. This was particularly evident when using it to
  change gcc/clang versions.

  Restructured the corresponding compiler settings to define default
  versions (eg, "default_gcc_version") that limits the scope of
  changes performed by foamConfigurePaths and makes it easier to
  understand if changing manually.