pkg-cmake: Skip compilation in installation step

With CMake build system, the "make install" command always compiles
before installing, to make sure that everything is up-to-date.

In Buildroot environment, this is quite useless, because the package
has always already been compiled, either at first build, or when
invoking "make <package>-rebuild".
In all cases, when it comes to staging or target installation step, the
package has just been compiled.

Using "make install/fast" - in order to skip the compilation step - is
therefore more appropriate, more consistent with what other build
systems do, and saves time when you need to rebuild your package.

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>