Teach Assembler test to generate main.s at build time

Use a custom command to generate the assembly source file at build time.
Also set CMAKE_VERBOSE_MAKEFILE so the test output contains all the
build rules.  These two changes will show the entire .c -> .s -> .o and
final link commands in the test output.

Tests/Assembler/CMakeLists.txt

@@ -1,5 +1,7 @@
 cmake_minimum_required (VERSION 2.6)
 project(Assembler)
+message("CTEST_FULL_OUTPUT ")
+set(CMAKE_VERBOSE_MAKEFILE 1)
 
 set(SRCS)
 
@@ -9,8 +11,13 @@ if("${CMAKE_GENERATOR}" MATCHES "Makefile")
   if(("${CMAKE_C_COMPILER_ID}" MATCHES "^(GNU|HP|SunPro|XL)$") OR ("${CMAKE_C_COMPILER_ID}" STREQUAL "Intel"  AND  UNIX))
     set(C_FLAGS "${CMAKE_C_FLAGS}")
     separate_arguments(C_FLAGS)
-    execute_process(COMMAND ${CMAKE_C_COMPILER} ${C_FLAGS} -S "${CMAKE_CURRENT_SOURCE_DIR}/main.c" -o "${CMAKE_CURRENT_BINARY_DIR}/main.s")
-    set(SRCS "${CMAKE_CURRENT_BINARY_DIR}/main.s")
+    set(SRCS main.s)
+    add_custom_command(
+      OUTPUT main.s
+      COMMAND ${CMAKE_C_COMPILER} ${C_FLAGS} -S ${CMAKE_CURRENT_SOURCE_DIR}/main.c -o main.s
+      DEPENDS main.c
+      VERBATIM
+      )
   endif(("${CMAKE_C_COMPILER_ID}" MATCHES "^(GNU|HP|SunPro|XL)$") OR ("${CMAKE_C_COMPILER_ID}" STREQUAL "Intel"  AND  UNIX))
 endif("${CMAKE_GENERATOR}" MATCHES "Makefile")