cmake: set CMAKE_BUILD_TYPE only if .git exists
distros intend to fine tune the CFLAGS and CXXFLAGS by themselves, see
- https://git.alpinelinux.org/abuild/tree/abuild.conf
- https://wiki.archlinux.org/title/CMake_package_guidelines#CMake_undesired_behaviors
- https://github.com/Debian/debhelper/blob/
5d1bb29841043d8e47ebbdd043e6cd086cad508e/lib/Debian/Debhelper/Buildsystem/cmake.pm#L16
so instead of setting a CMAKE_BUILD_TYPE when building from a
dist tarball, let's just leave it empty.
Signed-off-by: Kefu Chai <kchai@redhat.com>