Revert "Fix Linux platform detection"

This reverts commit e423c6fca63376f7ee60ce77660f2c9ec60c1f0f.
Revert "Fix Linux detection in CMake files"

This reverts commit 1b2ec23d0b1736fb3dbf9f32872864350ccf67ea.
Fix CheckLinuxCompiler.cmake path
Define LINUX variable on UNIX platforms