CMakeLists: arm: fixup lto option

This fixes an issue introduced in [1], we incorrectly append
the `USING_LTO` option even when it's not defined. This ensures we
check that it is defined first.

[1] #2457

Signed-off-by: Wilfred Mallawa <[email protected]>

Author: twilfredo

CMakeLists.txt

@@ -322,7 +322,11 @@ if(CMAKE_SYSTEM_NAME MATCHES "Linux")
             SET(FLTO_COMPILE_OPTS "-fno-lto")
             SET(FLTO_LINK_OPTS "-fno-lto")
         else()
-            SET(FLTO_COMPILE_OPTS "-flto" "-${USING_LTO}")
+            if (DEFINED USING_LTO)
+                SET(FLTO_COMPILE_OPTS "-flto" "-${USING_LTO}")
+            else()
+                SET(FLTO_COMPILE_OPTS "-flto")
+            endif()
             SET(FLTO_LINK_OPTS "-flto")
         endif()
         SET(CMAKE_SHARED_LIBRARY_LINK_C_FLAGS "")