Enable LTO support for Chatterino builds (#4258)

This is disabled by default, and can be enabled by passing `-DCHATTERINO_LTO=On` to your cmake invocation.

src/CMakeLists.txt

@@ -665,6 +665,12 @@ if (BUILD_APP)
                 DESTINATION share/icons/hicolor/256x256/apps
                 )
     endif ()
+
+    if(CHATTERINO_ENABLE_LTO)
+        message(STATUS "Enabling LTO for ${EXECUTABLE_PROJECT}")
+        set_property(TARGET ${EXECUTABLE_PROJECT}
+            PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
+    endif()
 endif ()
 
 if (USE_PRECOMPILED_HEADERS)
@@ -843,3 +849,9 @@ else ()
         )
     endif()
 endif ()
+
+if(CHATTERINO_ENABLE_LTO)
+    message(STATUS "Enabling LTO for ${LIBRARY_PROJECT}")
+    set_property(TARGET ${LIBRARY_PROJECT}
+                 PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
+endif()