Feature: [Actions / CMake] support for generic linux builds (#8641)

These bundles can be opened on any "modern" Linux machine with
a driver that SDL2 supports.

Machines needs at least glibc 2.15, which was released 10 years ago.
It is build with CentOS 7 as base, and only assumes the following
libraries are available on the system:
- libc
- libdl
- libgcc_s
- libpthread
- librt
- libstdc++

All other libraries the game depends on are bundled together with
the game, so users don't need any library installed to use this
bundle. The downside of course is that this increases the binary
size a bit: 30 MiB of libraries are in this bundle.

RPATH is used to make ld-linux find the folder libraries are
stored in; however, system libraries are always used before these,
in the assumption libraries on the user system are more up-to-date.

Using -DOPTION_PACKAGE_DEPENDENCIES=ON switches on packaging
of libraries in the "lib" folder. This requires CMake 3.16 to
be installed; otherwise it will fail.

CMakeLists.txt

@@ -167,6 +167,27 @@ if(APPLE)
     endif()
 endif()
 
+if(OPTION_PACKAGE_DEPENDENCIES)
+    if(NOT UNIX)
+        message(FATAL_ERROR "Can only package dependencies on Linux")
+    endif()
+    if(OPTION_INSTALL_FHS)
+        message(FATAL_ERROR "Cannot install in FHS folders when we are packaging dependencies")
+    endif()
+    if(${CMAKE_VERSION} VERSION_LESS "3.16.0")
+        message(FATAL_ERROR "OPTION_PACKAGE_DEPENDENCIES can only work with CMake 3.16+; you are using ${CMAKE_VERSION}")
+    endif()
+
+    # If we are packaging dependencies, we do two things:
+    # 1) set the RPATH to include $ORIGIN/lib; $ORIGIN (that literal string)
+    #    is a Linux indicator for "path where application is". In CMake, we
+    #    have to do this before add_executable() is executed.
+    # 2) copy the libraries that we compile against to the "lib" folder.
+    #    This is done in InstallAndPackage.cmake.
+    set(CMAKE_INSTALL_RPATH "\$ORIGIN/lib")
+    set(CMAKE_BUILD_WITH_INSTALL_RPATH ON)
+endif()
+
 include(SourceList)
 
 # Needed by rev.cpp