1048 - #elif without matching #if.

Description
A #elif token can only be used after an #if token or another #elif token.
Example
Config:
#elif __LINUX__ #endif
Output:
C:\Test\fbuild.bff(1,2): FASTBuild Error #1048 - #elif without matching #if. #elif __LINUX__ ^ \--here
Fix:
#if __OSX__ #elif __LINUX__ #endif