dlfcn required for windows build

[andybrucenet]

Describe the bug
When building on windows receive a compile error from src/common/ossl_helpers.c - dlfcn.h is not available on windows

To Reproduce
Try to build on windows :)

Expected behavior
One of four things:

1. Provide cross-platform code to load shared libraries (alternative to dlopen / dlclose)
2. #ifdef around #include <dlfcn.h> / dlfcn functions (simply ignore on windows and assume that required libraries are loaded).
3. Require dlfcn-win32 package with basic instructions on the liboqs github page on how to install (prerequisite).
4. Auto-install dlfcn-win32 from dlfcn-win32 github page as part of standard build.

Screenshots
I even put on a screenshot for you :)

Environment (please complete the following information):

• OS: Windows 11 with Visual Studio 2022 latest patches
• OpenSSL version: 3.3.0
• Compiler version used: MSVC (CL) 19.39.33523
• Build variables used: no overrides
• liboqs version: main branch

Additional context
I added logs collected from my run which shows all the configuration occurring.
buildliboqs.log''

I worked around the problem as follows:

// ABr: avoid dlfcn on windows
#if !defined(_MSC_VER)
#include <dlfcn.h>
#endif
[...]
static void ensure_symbol(const char *name, void **symp) {
#if !defined(_MSC_VER)
	if (!*symp) {
		void *sym = dlsym(libcrypto_dlhandle, name);
		if (!sym) {
			exit(EXIT_FAILURE);
		}
		*symp = sym;
	}
#endif
}
static void ensure_library(void) {
#if !defined(_MSC_VER)
	if (!libcrypto_dlhandle) {
		libcrypto_dlhandle = dlopen(OQS_OPENSSL_CRYPTO_SONAME,
		                            RTLD_LAZY | RTLD_LOCAL);
		if (!libcrypto_dlhandle) {
			exit(EXIT_FAILURE);
		}
	}

#define ENSURE_SYMBOL(name) \
    ensure_symbol(#name, (void **)&_oqs_ossl_sym_##name)
#define FUNC(ret, name, args, cargs)        \
    ENSURE_SYMBOL(name);
#define VOID_FUNC FUNC
#include "ossl_functions.h"
#undef VOID_FUNC
#undef FUNC
#undef ENSURE_SYMBOL
#endif
}

[baentsch]

Please check PLATFORMS.md: You'll notice limited Windows support for liboqs (we don't have people knowledgeable/interested in that platform for the project). Any contributions via PR welcome!

[andybrucenet]

Just saw this. I'll be happy to put in some code as soon as I get to a stopping point.

On the good news - I have integrated all the defined NIST algos for DSA / KEM on Windows (with my local hacks for now), macOS, iOS, Android, and Linux (glibc 2.34 since a "static" openssl build...still requires a glibc unless yet-more-work is done with musl).

All my tests pass on every platform. I still need to prove interoperability such that when keys are exchanged between different platforms via PEM files that everything works. But so far things is looking good.