Hack-fix build breakage on Linux/GCC from fc4856f9

For some reason, the new `GccIsClang` variable introduced via
fc4856f9e811d9a23ae9212f43a09ddf5ef12b26 for addressing #8148
isn't set explicitly to `NO`; so this simply changes the test
`ifeq $(GccIsClang) NO` to `ifneq $(GccIsClang) YES` which
should fix the build with and w/o Clang.