Fix a thinko in r232322, where gcc (and its tools) are not built during the cross-tools stage, if CC=clang and WITH_CLANG_IS_CC is not set.
Fix a thinko in r232322, where gcc (and its tools) are not built during
the cross-tools stage, if CC=clang and WITH_CLANG_IS_CC is not set.

This causes no 'cc' to be installed in the temporary cross-tools tree,
making lint fall over later in the build, because it ignores ${CC} and
attempts to run 'cc' anyway.

To fix this, only skip building gcc during cross-tools, if WITHOUT_GCC
is set, or if WITH_CLANG_IS_CC is set.

Pointy hat to:	dim
MFC after:	2 weeks

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase