CMake: Add support for "browse" mode
CMake: Add support for "browse" mode

Fixes ninja-build/ninja#1822, fixes ninja-build/ninja#1853

Adds support for `ninja -t browse` to CMake builds.

The platform support logic is copied from configure.py, so Windows,
Solaris and AIX are treated as 'unsupported' platforms. All other
platforms are assumed to be supported.

As discussed in #1853, when built via CMake the `ninja` executable
looks for a binary called `python` in the current path, in order to
launch the "browse" mode. The behaviour differs from that of the
configure.py script, which looks for a python executable that has the
*same name* as the python executable that invoked the configure script.

maruel/ninja,iwadon/ninja,sgraham/ninja,bradking/ninja,iwadon/ninja,maruel/ninja,sgraham/ninja,AoD314/ninja,AoD314/ninja,ninja-build/ninja,Qix-/ninja,maruel/ninja,ninja-build/ninja,ninja-build/ninja,Qix-/ninja,Qix-/ninja,bradking/ninja,ninja-build/ninja,maruel/ninja,iwadon/ninja,Qix-/ninja,bradking/ninja,AoD314/ninja,bradking/ninja,AoD314/ninja,sgraham/ninja,sgraham/ninja,iwadon/ninja