cmake: Ensure git exists before executing it

cmake: Ensure git exists before executing it

CMake 3.28 has turned stricter when executing string(REPLACE …) and
expects four or more commands. In case of distro package builds from
tarball, it happens that git is not present. CTags.cmake tries to
catch that by veriying the exit status of the command, but as there
is in fact git | awk, awk returns 0 even when git does not exist.

Ensure that the variable submodules has been defined before trying
to replace substrings in this variable.

Signed-off-by: Dominique Leuenberger <dimstar@opensuse.org>