checking for gcc... gcc
  ) works... nor the C compiler (gcc
configure: error: installation or configuration problem: C compiler cannot creat
e executables.
*** Fix above errors and then restart with "make -f client.mk build"

This is a problem that .mozconfig somehow has got windows line endings instead of unix line-endings.
In that case, you need to do "dos2unix.exe .mozconfig" to fix the error.