CMake: Actually set the LAGOM_USE_LINKER option automatically

When this feature was finally merged, the serenity_option in
lagom_options.cmake had the unintended side effect of always setting the
cache variable to "" in the initial cache. In order to actually set the
linker to use to be lld or mold, we need to set with the FORCE flag in
the use_linker.cmake file.

Found by checking with the CMake variable_watch() function.

This will stop the build from spamming the "Using LLD to link Lagom"
message all over the place :^)

Meta/CMake/use_linker.cmake

@@ -4,21 +4,21 @@
 # SPDX-License-Identifier: BSD-2-Clause
 #
 
-if (NOT APPLE AND "${LAGOM_USE_LINKER}" STREQUAL "")
+if (NOT APPLE AND NOT LAGOM_USE_LINKER)
     find_program(LLD_LINKER NAMES "ld.lld")
     if (LLD_LINKER)
-        message("Using LLD to link Lagom.")
-        set(LAGOM_USE_LINKER "lld" CACHE STRING "")
+        message(STATUS "Using LLD to link Lagom.")
+        set(LAGOM_USE_LINKER "lld" CACHE STRING "" FORCE)
     else()
         find_program(MOLD_LINKER NAMES "ld.mold")
         if (MOLD_LINKER)
-            message("Using mold to link Lagom.")
-            set(LAGOM_USE_LINKER "mold" CACHE STRING "")
+            message(STATUS "Using mold to link Lagom.")
+            set(LAGOM_USE_LINKER "mold" CACHE STRING "" FORCE)
         endif()
     endif()
 endif()
 
-if (NOT "${LAGOM_USE_LINKER}" STREQUAL "")
+if (LAGOM_USE_LINKER)
     set(LINKER_FLAG "-fuse-ld=${LAGOM_USE_LINKER}")
     set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${LINKER_FLAG}")
     set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} ${LINKER_FLAG}")