cmake: allow pkg-config in more envs

Before this patch, `pkg-config` was used for `UNIX` builds only (with
a few exceptions like wolfSSL, libssh, gsasl, libuv). This patch extends
`pkg-config` use to all envs except: `MSVC` without vcpkg. Meaning MSVC
with vcpkg will now use it. Also mingw on Windows.

Also apply the new condition to options where `pkg-config` was used
unconditionally (= for all targets). These are:
`-DCURL_USE_WOLFSSL=ON`, `-DCURL_USE_LIBSSH=ON`,
`-DCURL_USE_GSASL=ON` and `-DCURL_USE_LIBUV=ON`

This patch may still cause regressions for cross-builds (e.g. mingw
cross-build from Unix) and potentially other cases. If that happens, we
recommend using some of these methods to explicitly disable `pkg-config`
when using CMake:
- CMake option: `-DPKG_CONFIG_EXECUTABLE=`
  (or `-DPKG_CONFIG_EXECUTABLE=nonexistent` or similar)
  This is similar to the (curl-specific) `PKG_CONFIG` env for autotools.
- export env: `PKG_CONFIG_LIBDIR=`
  (or `PKG_CONFIG_PATH`, `PKG_CONFIG_SYSROOT_DIR`,
  or the CMake-specific `PKG_CONFIG`)

We may improve control over this in a future patch, also allowing opting
in MSVC (without vcpkg).

Ref: #14405
Ref: #14408
Ref: #14140
Closes #14483

CMake/FindNGHTTP2.cmake

@@ -30,7 +30,7 @@
 # NGHTTP2_LIBRARIES     The libraries needed to use nghttp2
 # NGHTTP2_VERSION       Version of nghttp2
 
-if(UNIX)
+if(NOT MSVC OR VCPKG_TOOLCHAIN)
   find_package(PkgConfig QUIET)
   pkg_search_module(PC_NGHTTP2 "libnghttp2")
 endif()