MDEV-22864: cmake/libutils account for cmake-2.8.12.1

That doesn't support STRING(APPEND ..)

cmake/libutils.cmake

@@ -207,10 +207,9 @@ MACRO(MERGE_STATIC_LIBS TARGET OUTPUT_NAME LIBS_TO_MERGE)
 
       SET(SCRIPT_CONTENTS "CREATE $<TARGET_FILE:${TARGET}>\n")
       FOREACH(LIB ${STATIC_LIBS})
-        STRING(APPEND SCRIPT_CONTENTS "ADDLIB ${LIB}\n")
+        SET(SCRIPT_CONTENTS "${SCRIPT_CONTENTS}\nADDLIB ${LIB}\n")
       ENDFOREACH()
-      STRING(APPEND SCRIPT_CONTENTS "SAVE\nEND\n")
-      FILE(WRITE ${MRI_SCRIPT_TPL} "${SCRIPT_CONTENTS}")
+      FILE(WRITE ${MRI_SCRIPT_TPL} "${SCRIPT_CONTENTS}\nSAVE\nEND\n")
       FILE(GENERATE OUTPUT ${MRI_SCRIPT} INPUT ${MRI_SCRIPT_TPL})
 
       ADD_CUSTOM_COMMAND(TARGET ${TARGET} POST_BUILD