cmake: fix install dirs reversing at second build

CMakeLists.txt

@@ -48,6 +48,9 @@ if(FXLIBC_TARGET STREQUAL gint)
   # Default to fxSDK install path
   if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
     set(CMAKE_INSTALL_PREFIX "${FXSDK_COMPILER_INSTALL}" CACHE PATH "..." FORCE)
+  endif()
+
+  if(CMAKE_INSTALL_PREFIX STREQUAL "${FXSDK_COMPILER_INSTALL}")
     set(LIBDIR ".")
     set(INCDIR "include")
   endif()