Use add_definitions() in CMake rather than #cmakedefine in C source.
don't gitignore src/extern.h anymore.
clean split of global variables declaration and definition.

  All globales variables should be declared in header files with the
  `extern' keyword and defined in one place (currently the main file
  dfc.c)
Merge pull request #1 from kAworu/extern-header-cleanup

extern.h header cleanup