This moves setting of compilation flags to common file to be used by all
subprojects and explicitly enables only C++ support, skipping whole C
support in CMake.

Replace with a placeholder to be filled by external tools

[ci skip]

for Linux software galleries.

This allows using CMake-standard `BUILD_SHARED_LIBS` flag to dynamically
set how libopenrct2 should be built.

`cmake -DBUILD_SHARED_LIBS=OFF` will result in `openrct2` statically
linked with `libopenrct2`

Fixes #5918.

It is easier to add the compression manually when desired as to remove
it in all other cases.

the project definition is used by cmake to fill the subfolder for
${CMAKE_INSTALL_DOCDIR}.

at least on Linux this makes the build system easily adaptable to the
different distributions. GNUInstallDirs was added in cmake 2.8 so the
version is increades as well.
Testing on all other platforms is necessary, I have only a Linux system
at hand right now.

RENAME was included in the quotes in a previous commit, so cmake treated
it as part of the destination path.

Which set rpath to $ORIGIN so it can be run out the box.