Experimental spec2def() "improvements"

[HBelusca]

Purpose

The main purpose of this PR is to experiment with the modification of the spec2def() command so as to associate it with a build target.

This allows associating automatically the generated .def and _stubs.c files to the list of source files for the build target in question, and setting up optional additional linker options.

Note: Parts of commit https://github.com/reactos/reactos/commit/a5ec9a07aa35269e7cafcf6eaabcc18d14bb26a8 are modified.

Proposed changes

[SPEC2DEF] Parse the .spec only once, and write to the output files all at once.

The chosen approach is to keep an array of opened output files with
corresponding write handlers, so that ParseFile() is called only once
and for each export the corresponding lines are written in all the
output files at once.

Another approach would have been to parse the input file once and create
a list of EXPORT structures in memory. Then the output files would have
been opened successively and lines would have been written for each
cached EXPORT structure.

[CMAKE][SPEC2DEF] Modify the spec2def() command so as to associate it with a build target.

This allows associating automatically the generated .def and _stubs.c
files to the list of source files for the build target in question, and
setting up optional additional linker options.

Note that the additional import library targets are still generated
separately on-demand.

[REACTOS] Adjust the spec2def() calls in the CMakeLists.txt files.