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.

cmake/InstallAndPackage.cmake

@@ -123,8 +123,12 @@ elseif(UNIX)
     # With FHS, we can create deb/rpm/... Without it, they would be horribly broken
     # and not work. The other way around is also true; with FHS they are not
     # usable, and only flat formats work.
-    if(NOT OPTION_INSTALL_FHS)
+    if(OPTION_PACKAGE_DEPENDENCIES)
         set(CPACK_GENERATOR "TXZ")
+        set(PLATFORM "generic")
+    elseif(NOT OPTION_INSTALL_FHS)
+        set(CPACK_GENERATOR "TXZ")
+        set(PLATFORM "unknown")
     else()
         find_program(LSB_RELEASE_EXEC lsb_release)
         execute_process(COMMAND ${LSB_RELEASE_EXEC} -is
@@ -149,7 +153,7 @@ elseif(UNIX)
             string(REGEX MATCH "ID=(.*)" _ ${OS_RELEASE_CONTENTS})
             set(DISTRO_ID ${CMAKE_MATCH_1})
             if(DISTRO_ID STREQUAL "arch")
-                set(PLATFORM "generic")
+                set(PLATFORM "arch")
                 set(CPACK_GENERATOR "TXZ")
             else()
                 set(UNSUPPORTED_PLATFORM_NAME "Linux distribution '${DISTRO_ID}' from /etc/os-release")
@@ -163,12 +167,39 @@ elseif(UNIX)
             set(CPACK_GENERATOR "TXZ")
             message(WARNING "Unknown ${UNSUPPORTED_PLATFORM_NAME} found for packaging; can only pack to a txz. Please consider creating a Pull Request to add support for this distribution.")
         endif()
-
-        set(CPACK_PACKAGE_FILE_NAME "openttd-#CPACK_PACKAGE_VERSION#-linux-${PLATFORM}-${CPACK_SYSTEM_NAME}")
     endif()
 
+    set(CPACK_PACKAGE_FILE_NAME "openttd-#CPACK_PACKAGE_VERSION#-linux-${PLATFORM}-${CPACK_SYSTEM_NAME}")
+
 else()
     message(FATAL_ERROR "Unknown OS found for packaging; please consider creating a Pull Request to add support for this OS.")
 endif()
 
 include(CPack)
+
+if(OPTION_PACKAGE_DEPENDENCIES)
+    # Install all dependencies we can resolve, with the exception of ones that
+    # every Linux machine should have. This should make this build as generic
+    # as possible, where it runs on any machine with the same or newer libc
+    # than the one this is compiled with.
+    # We copy these libraries into lib/ folder, so they can be found on game
+    # startup. See comment in root CMakeLists.txt for how this works exactly.
+    install(CODE [[
+        file(GET_RUNTIME_DEPENDENCIES
+                RESOLVED_DEPENDENCIES_VAR DEPENDENCIES
+                UNRESOLVED_DEPENDENCIES_VAR UNRESOLVED_DEPENDENCIES
+                EXECUTABLES openttd
+                POST_EXCLUDE_REGEXES "ld-linux|libc.so|libdl.so|libm.so|libgcc_s.so|libpthread.so|librt.so|libstdc...so")
+        file(INSTALL
+                DESTINATION "${CMAKE_INSTALL_PREFIX}/lib"
+                FILES ${DEPENDENCIES}
+                FOLLOW_SYMLINK_CHAIN)
+
+        # This should not be possible, but error out when a dependency cannot
+        # be resolved.
+        list(LENGTH UNRESOLVED_DEPENDENCIES UNRESOLVED_LENGTH)
+        if(${UNRESOLVED_LENGTH} GREATER 0)
+            message(FATAL_ERROR "Unresolved dependencies: ${UNRESOLVED_DEPENDENCIES}")
+        endif()
+    ]])
+endif()