cmake: raise a fatal error when C compiler is not found

This commit introduces an early error in case a valid C compiler is not
found in the system.

This will help to early identify misconfigured systems with the error:
  C compiler <compiler> not found - Please check your toolchain
  installation

instead of an obscure error, such as:
No such file or directory: LIBGCC_FILE_NAME: ''

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
1 file changed