Add REMOVE_ABSOLUTE_PATHS CMake option

This option is enabled by default and will replace absolute paths of all
source directory file paths with relative paths in the compiled binary,
if the compiler supports it. Of course, this isn't needed if you compile
with all paths removed anyways (e.g. in Release mode).

The purpose is to help make builds reproducible and to remove any
potentially sensitive information about the user or the user's system
from the compiled binary.

Both Clang and GCC support -fdebug-prefix-map, -fmacro-prefix-map, and
-ffile-prefix-map. In particular, -ffile-prefix-map is just a flag that
does both -fdebug-prefix-map and -fmacro-prefix-map.

According to https://reproducible-builds.org/docs/build-path/ ,
-fdebug-prefix-map is available in all GCC versions but only available
starting from Clang 3.8, and -fmacro-prefix-map and -ffile-prefix-map
are available since GCC 8 and Clang 10. So we check the compiler version
and use the available flags depending on if the compiler supports it or
not.

This does make debugging a bit more annoying, but there are a couple
ways to rectify this. Either disable it with
-DREMOVE_ABSOLUTE_PATHS=OFF, or add a `.gdbinit` that consists of

    set substitute-path . ../..

so that `.` is considered to be `../..`. Of course, if you need to,
replace `../..` with the actual source directory path (in my case it's
`../../..` because I place my build folders in another subdirectory to
have multiple build folders in one directory).

This doesn't need to be a global `.gdbinit`, it can be in a
directory-specific `.gdbinit` (similar to how `.gitignore`s can also be
directory-specific). But then you need to add `add-auto-load-safe-path`
to your `.gdbinit` to load any directory-specific `.gdbinit`s.

The above is for GDB; I don't know what (if anything) needs to be done
for LLDB; I don't use LLDB.

Fixes #889.

desktop_version/CMakeLists.txt

@@ -24,6 +24,9 @@ if(OFFICIAL_BUILD AND NOT MAKEANDPLAY)
     set(GOG ON)
 endif()
 
+option(REMOVE_ABSOLUTE_PATHS "If supported by the compiler, replace all absolute paths to source directories compiled into the binary (if any) with relative paths" ON)
+
+
 # Architecture Flags
 if(APPLE)
     # Wow, Apple is a huge jerk these days huh?
@@ -329,6 +332,41 @@ if(MSVC)
 endif()
 
 
+if(REMOVE_ABSOLUTE_PATHS)
+    if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+        if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 3.8)
+            set(SUPPORTS_DEBUG_PREFIX_MAP TRUE)
+        else()
+            set(SUPPORTS_DEBUG_PREFIX_MAP FALSE)
+        endif()
+        if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 10.0)
+            set(SUPPORTS_FILE_PREFIX_MAP TRUE)
+        else()
+            set(SUPPORTS_FILE_PREFIX_MAP FALSE)
+        endif()
+    elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+        set(SUPPORTS_DEBUG_PREFIX_MAP TRUE)
+        if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 8.0)
+            set(SUPPORTS_FILE_PREFIX_MAP TRUE)
+        else()
+            set(SUPPORTS_FILE_PREFIX_MAP FALSE)
+        endif()
+    else()
+        set(SUPPORTS_DEBUG_PREFIX_MAP FALSE)
+        set(SUPPORTS_FILE_PREFIX_MAP FALSE)
+    endif()
+
+    get_filename_component(REPO_DIR ../ ABSOLUTE)
+
+    # Remove absolute source paths from compiled binary
+    if(SUPPORTS_FILE_PREFIX_MAP)
+        list(APPEND GLOBAL_COMPILE_FLAGS -ffile-prefix-map=${REPO_DIR}=.)
+    elseif(SUPPORTS_DEBUG_PREFIX_MAP)
+        list(APPEND GLOBAL_COMPILE_FLAGS -fdebug-prefix-map=${REPO_DIR}=.)
+    endif()
+endif()
+
+
 target_compile_options(VVVVVV PRIVATE ${GLOBAL_COMPILE_FLAGS})
 
 foreach(static_library IN LISTS STATIC_LIBRARIES)