1502 - LightCache only compatible with MSVC Compiler.

Description
The LightCache is currently only supported when using the MSVC Compiler. This error will be generated if using any other compiler.
Example
Config:
Compiler( 'compiler' ) { .Executable = 'gcc' .UseLightCache_Experimental = true }
Output:
c:\test\fbuild.bff(1,1): FASTBuild Error #1502 - Compiler() - LightCache only compatible with MSVC Compiler. Compiler( 'compiler' ) ^ \--here
Fix:
Compiler( 'compiler' ) { .Executable = 'gcc' }