Please do not simply revert everything if it doesn't compile.
Fix the problem with #ifdef OS_LINUX #endif clauses instead.

I merged all "OS-specific" buildscripts and makefiles. Now you only have
to modify 1 file instead of 6 (1 per OS) when you change the build system.

The difference between Unix-derived operating systems is really
not big enough to jusify 6 versions of every script and makefile.
We don't need 71500 lines of "copy-paste" code to handle those small differences.