[CMake] Fix dependencies for generating mem_initializer.cpp.

Previously CMake was not aware of which headers files the generation
of `mem_initializer.cpp` depended on. Consequently this could result
in broken incremental builds if

* Existing headers that declare memory initializers/finalizers change.
* New headers are added that declare memory initializers/finalizer.

Now the `z3_add_component()` CMake function has been modifed so that
it now takes an optional `MEMORY_INIT_FINALIZER_HEADERS` argument
which allows the headers that declare memory initializers/finalizers
to be explicitly listed.

With this information CMake will now regenerate `mem_initializer.cpp`
correctly.

This required the `mk_mem_initializer_cpp_internal()` function to be
changed to take a list of header files rather than a list of component
source directories. The two consumers (CMake and Python/Makefile build
systems) of this function have been modified to work with this change.

This partially fixes #1030.

src/util/CMakeLists.txt

@@ -61,4 +61,11 @@ z3_add_component(util
     z3_exception.cpp
   EXTRA_REGISTER_MODULE_HEADERS
     env_params.h
+  MEMORY_INIT_FINALIZER_HEADERS
+    debug.h
+    gparams.h
+    prime_generator.h
+    rational.h
+    symbol.h
+    trace.h
 )