build: make HAVE_LIBCRYPT32 local to wincng.c

libssh2 uses `wincrypt.h` aka the `crypt32` Windows system library
for the function `CryptDecodeObjectEx()` [1]. This function has been
available for Win32 (and UWP/WinRT apps) for a long while. Even old
MinGW supports it, and also Watcom 1.9, of the rare/old compilers
I checked.

CMake had it permanently enabled, while it also did an extra check
for the header to add the lib to the lib list. Autotools did the
detection proper. Other builds had it permanently enabled.

It seems safe to assume this function/header/lib is available in all
environments we support.

In this patch we simplify by deleting these detections and feature
flags from all build tools.

Keep the feature flag internal to `wincng.h`, and for extra safety add
the new macro `LIBSSH2_WINCNG_DISABLE_WINCRYPT` do disable it via
custom `CPPFLAGS`.

WinCNG's other requirement is `bcrypt`. That also has been universally
available for a long time. Here the only known outlier is old/legacy
MinGW, which is missing support.

[1] https://learn.microsoft.com/en-us/windows/win32/api/wincrypt/nf-wincrypt-cryptdecodeobjectex

Closes #916

os400/libssh2_config.h

@@ -77,9 +77,6 @@
 /* use Ioctlsocket() for non-blocking sockets */
 #undef HAVE_IOCTLSOCKET_CASE
 
-/* Define if you have the crypt32 library. */
-#undef HAVE_LIBCRYPT32
-
 /* Define if you have the gcrypt library. */
 #undef HAVE_LIBGCRYPT