Identify default mingw runtime (mingw builds only).

See #21612 for context.

Basically, by setting $Config{libc} to the default runtime (which will
be either -lucrt or -lmsvcrt) we enable perl scripts to determine the
identity of the default runtime.

.mailmap

@@ -790,6 +790,7 @@ Simon Schubert <corecode@fs.ei.tum.de> Simon 'corecode' Schubert <corecode@fs.ei
 Sisyphus <sisyphus@cpan.org> <sisyphus359@gmail.com>
 Sisyphus <sisyphus@cpan.org> Sisyphus <sisyphus1@optusnet.com.au>
 Sisyphus <sisyphus@cpan.org> sisyphus <sisyphus1@optusnet.com.au>
+Sisyphus <sisyphus@cpan.org> sisyphus <sisyphus359@gmail.com>
 Sisyphus <sisyphus@cpan.org> sisyphus <sisyphus@cpan.org>
 Slaven Rezic <slaven@rezic.de> <slaven.rezic@berlin.de>
 Slaven Rezic <slaven@rezic.de> eserte@vran.herceg.de <eserte@vran.herceg.de>

MANIFEST

@@ -6710,6 +6710,7 @@ win32/config_H.gc			Win32 config header (MinGW build)
 win32/config_h.PL			Perl code to convert Win32 config.sh to config.h
 win32/config_H.vc			Win32 config header (Visual C++ build)
 win32/config_sh.PL			Perl code to update Win32 config.sh from Makefile
+win32/configure/rt.c			identify default runtime
 win32/create_perllibst_h.pl		creates perllibst.h file for inclusion from perllib.c
 win32/distclean.bat			Remove _ALL_ files not listed here in MANIFEST
 win32/fcrypt.c				crypt() implementation

win32/config_sh.PL

@@ -155,6 +155,9 @@ if (exists $opt{cc}) {
     }
     elsif ($opt{cc} =~ /\bgcc\b/) {
         chomp($opt{gccversion} = `$opt{cc} -dumpversion`);
+        chomp($opt{libc} = `$opt{cc} -o rt.exe configure/rt.c 1>nul 2>&1 && rt`);
+        if(-e 'rt.exe') { unlink 'rt.exe' } # rt.exe no longer needed
+        else { die "Failed to identify default runtime" }
     }
 }
 

win32/configure/rt.c

@@ -0,0 +1,15 @@
+/*
+ Print the default runtime. $Config{libc}
+ will be set to this specified value.
+*/
+#include <stdio.h>
+#include <stddef.h>
+
+int main(void) {
+#if defined(_UCRT)
+  printf("-lucrt\n");
+#else
+  printf("-lmsvcrt\n");
+#endif
+  return 0;
+}