Remove hardcoded optimisation option.
Disable ccache detection.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -31,7 +31,6 @@ ENDIF(NOT CMAKE_BUILD_TYPE)
 message(STATUS "Build type CMAKE_BUILD_TYPE set to ${CMAKE_BUILD_TYPE}")
 
 include(UseCompilerCache)
-find_compiler_cache(PROGRAM ccache)
 
 list(APPEND GODOT_CPP_CMAKE_VARS "CMAKE_BUILD_TYPE;CMAKE_CONFIGURATION_TYPES;CMAKE_TOOLCHAIN_FILE;PYTHON_EXECUTABLE;Python3_EXECUTABLE")
 get_cmake_property(vars CACHE_VARIABLES)
@@ -518,7 +517,6 @@ target_compile_options(freeorionparseobj
     PRIVATE
         $<$<CXX_COMPILER_ID:Clang>:-ftemplate-depth=512>
         $<$<CXX_COMPILER_ID:AppleClang>:-ftemplate-depth=512>
-        $<$<AND:$<NOT:$<BOOL:${BUILD_TESTING}>>,$<CXX_COMPILER_ID:GNU>>:-O3>
 )
 
 target_compile_definitions(freeorionparseobj
