Link against debug or release UCRT DLLs depending on $debug
Removed unused -pthread flag (caused compiler warning)
Fix CFLAGS, as AC_PROG_CC will not detect Clang in MSVC mode as a GNU C compiler and therefore sets CFLAGS to just -g.
Fixes incorrect import statement in Testing.h, which caused linker errors when running tests on Windows MSVC because the compiler needs to see the __dllspec on NSCalendarDate in order to be able to link against it.
A couple of fixes for Windows MSVC:
-pthread
flag (caused compiler warning)-g
.