Implements [Feature #3456]: Support pkgconf on windows

1. Store the `PKG_CONFIG` variable in Makefile.sub (or try to get it from the ENV var PKG_CONFIG in mkmf.rb)
2. Try to use --msvc-syntax, with a fallback to replacing -Lxxx with -libpath:xxx. --msvc-syntax has been in pkgconf since 1.4.0 (released 7 years ago). pkg-config (freedesktop), does not support it, hence the fallback.
3. The `try_ldflags` passes these `ldflags` as the `opt` parameter to the `link_command`, not as `ldflags`. Unix systems are forgiving in that regard, MSVC is not: as a result as passing them as `opt`, they (specifically the `/libpath:xxx` ones) end up passed before the `-link` command to `cl.exe` and it throws because it ignores it and therefore can't find the lib.

```
cl : Command line warning D9002 : ignoring unknown option '-libpath:C:/Users/julien/.conan2/p/libff3726d89a6255c/p/lib'
```

lib/mkmf.rb

@@ -604,9 +604,9 @@ MSG
     yield(opt, opts)
   end
 
-  def try_link0(src, opt = "", **opts, &b) # :nodoc:
+  def try_link0(src, opt = "", ldflags: "", **opts, &b) # :nodoc:
     exe = CONFTEST+$EXEEXT
-    cmd = link_command("", opt)
+    cmd = link_command(ldflags, opt)
     if $universal
       require 'tmpdir'
       Dir.mktmpdir("mkmf_", oldtmpdir = ENV["TMPDIR"]) do |tmpdir|
@@ -750,7 +750,7 @@ MSG
 
   # :nodoc:
   def try_ldflags(flags, werror: $mswin, **opts)
-    try_link(MAIN_DOES_NOTHING, flags, werror: werror, **opts)
+    try_link(MAIN_DOES_NOTHING, "", ldflags: flags, werror: werror, **opts)
   end
 
   # :startdoc:
@@ -1968,7 +1968,7 @@ SRC
       if pkgconfig = with_config("#{pkg}-config") and find_executable0(pkgconfig)
       # if and only if package specific config command is given
       elsif ($PKGCONFIG ||=
-             (pkgconfig = with_config("pkg-config") {config_string("PKG_CONFIG") || "pkg-config"}) &&
+             (pkgconfig = with_config("pkg-config") {config_string("PKG_CONFIG") || ENV["PKG_CONFIG"] || "pkg-config"}) &&
              find_executable0(pkgconfig) && pkgconfig) and
            xsystem([*envs, $PKGCONFIG, "--exists", pkg])
         # default to pkg-config command
@@ -1980,11 +1980,34 @@ SRC
         pkgconfig = nil
       end
       if pkgconfig
+        has_ms_win_syntax = false
+        if $mswin
+          has_ms_win_syntax = xpopen([pkgconfig, "--help"]).read.include?('msvc-syntax')
+          if has_ms_win_syntax
+            args << "--msvc-syntax"
+          else
+            Logging.message("WARNING: #{pkgconfig} does not support the --msvc-syntax. Try using a recent pkgconf instead")
+          end
+        end
         get = proc {|opts|
           opts = Array(opts).map { |o| "--#{o}" }
           opts = xpopen([*envs, pkgconfig, *opts, *args], err:[:child, :out], &:read)
           Logging.open {puts opts.each_line.map{|s|"=> #{s.inspect}"}}
-          opts.strip if $?.success?
+          if $?.success?
+            opts = opts.strip
+            if $mswin and not has_ms_win_syntax
+              opts = Shellwords.shellwords(opts).map { |s|
+                if s.start_with?('-l')
+                  "#{s[2..]}.lib"
+                elsif s.start_with?('-L')
+                  "/libpath:#{s[2..]}"
+                else
+                  s
+                end
+              }.quote.join(" ")
+            end
+            opts
+          end
         }
       end
       orig_ldflags = $LDFLAGS

win32/Makefile.sub

@@ -1168,6 +1168,7 @@ s,@top_srcdir@,$(srcdir),;t t
 s,@try_header@,try_compile,;t t
 s,@ruby_pc@,$(ruby_pc),;t t
 s,@RJIT_SUPPORT@,$(RJIT_SUPPORT),;t t
+s,@PKG_CONFIG@,$(PKG_CONFIG),;t t
 <<KEEP
 
 !if "$(HAVE_BASERUBY)" != "yes" || "$(CROSS_COMPILING)" == "yes"