Add CEXTRA AND CXXEXTRA fields so we can pass -mno-cygwin to winegcc

Add CEXTRA AND CXXEXTRA fields so we can pass -mno-cygwin to winegcc
but not to wrc which chokes on it. Add RCEXTRA for symetry and for the
user.
Remove T_INIT and get_default_init(). These are obsolete (used to
select the entry-point, WinMain or main).
If the directory contains headers, then add '.' to INCLUDE_PATH.
Specify -mnocygwin during the link stage if we are to link with the
msvcrt.
Transform XXX_APPMODE into XXX_LDFLAGS for more flexibility.
Correctly pass '-mconsole' or '-mwindows' to the link stage.
Remove XXX_BASEMODULE, XXX_SPEC_SRCS and SPEC_SRCS. They are
obsolete.
Add implicit build rules for .c, .cpp, .cxx files so that our settings
(e.g. INCLUDE_PATH) are used.
Fix the rule for building RC files (it was invalid and rejected by
make). Convert it to an implicit rule like the others.
Add rules for 'make clean'.
Add the missing rules for recursive compilation.
Remove obsolete elements from the link command (LDDLLFLAGS,
ALL_LIBRARY_PATH, LIBS).