Always build crt1.o, dllcrt1.o, crt2.o and dllcrt2.o.
Create a libcoldname.a for the oldname library for CRTDLL.
Restrict libmoldname.a for the oldname library for MSVCRT.
* configure.in: eliminate setting RUNTIME and CRT_ID variables.
Restructure the $target_os case logic.
Always name the MinGW thread dll helper mingwm.
Change Cygwin's HEADER_SUBDIR value from mingw32 to mingw.
* configure: regenerate.