When not available in the system, those libraries can be downloaded
from Wrap DB and built as subprojects.

This checks on Windows whether we have __USE_MINGW_ANSI_STDIO
defined for MinGW builds or whether we are building with Visual Studio
2015 or later, as sscanf(..., "%2hhx", ...) would either fail to build
or will fail to run correctly if either of these conditions do not hold
on Windows, as the stock Windows (pre-Visual Studio 2015) CRT does not
support the 'hh' modifier, as it is C99.

If the sscanf() implementation on Windows does not support the 'hh'
modifier, use a workaround so that we obtain the correct values we need
from sscanf().

I was wrong to think that the workaround is not needed anymore.

Fixes the following error:
libsoup-2.66.0/libsoup/soup-auth-ntlm.c:815:23: error: unknown conversion type character 'h' in format [-Werror=format=]
 sscanf(hex_pos, "%2hhx", &hmac[count]);
                     ^

This fixes issues when adding new symbols and building with
introspection enabled. Additionally, the workaround for
tests seems to be unneeded.

The proxy test uses apache, so it shouldn't run in parallel (see 5c1eea7).
For some reason parallelism wasn't disabled for this test, causing random
failures in CI.

Fixes #142