CMake compiler frontend variant and install-prefix

[lndpj]

Changes

• separate GitDescribe and ERICWTOOLS_VERSION
• add --always to git describe
• use add_definitions(-DERICWTOOLS_VERSION="${PROJECT_VERSION}-${GIT_DESCRIBE}")
• now the version output is ---- qbsp / ericw-tools 2.0.0-817c410d ----
• use CMAKE_CXX_COMPILER_FRONTEND_VARIANT
• add frontend specific optimization
• require C++23 standard
• use C23 standard but don't require it because of MSVC
• include(GNUInstallDirs) if not using VisualStudio
• add BUILD_INTERFACE to include_directories
• add VERSION, HOMEPAGE_URL and DESCRIPTION to PROJECT
• add ENABLE_PREFIX_INSTALL

Tested

• LibreQuake build
• ./build-linux-64.sh && cd build-linux && make install
• out of source-tree build

mkdir ericw-tools-build && cd ericw-tools-build
cmake ../ericw-tools -DENABLE_PREFIX_INSTALL=ON --install-prefix=/usr -DSKIP_TBB_INSTALL=ON
make install