6.2.0 build failure on x86_64 Skylake PC - FIX

[tsurzin]

This change worked to build test and run a version of GMP-6.2.0 for my PC.
The PC is running Msys2 under Windows 10 and without change GMP failed 
to build.

The following change and:
     autoconf
     ./configure
     make
     make check
     make install
built a working static version of GMP for my x86_64 Skylake PC.
With very limited understanding of the Gnu-tools I was unable to make a 
shared (.dll) version although Msys2 provides a generic one.

$ diff -u old/configure.ac gmp-6.2.0/configure.ac
--- old/configure.ac    2020-05-30 11:42:19.002324200 -0500
+++ gmp-6.2.0/configure.ac      2020-05-30 11:43:57.507601600 -0500
@@ -2011,7 +2011,7 @@
             # Sun cc.
             cc_64_cflags="-xO3 -m64"
             ;;
-         *-*-mingw* | *-*-cygwin)
+         *-*-mingw* | *-*-cygwin | *-*-msys )
             limb_64=longlong
             CALLING_CONVENTIONS_OBJS_64=""
             AC_DEFINE(HOST_DOS64,1,[Define to 1 for Windos/64])
@@ -2037,7 +2037,7 @@
  # x86s, but allow any CPU here so as to catch "none" too.
  #
  case $host in
-  *-*-mingw*)
+  *-*-mingw* | *-*-msys )
      gcc_cflags_optlist="$gcc_cflags_optlist nocygwin"
      gcc_cflags_nocygwin="-mno-cygwin"
      ;;
@@ -2664,7 +2664,7 @@

  AC_SUBST(LIBGMP_DLL,0)
  case $host in
-  *-*-cygwin* | *-*-mingw* | *-*-pw32* | *-*-os2*)
+  *-*-cygwin* | *-*-mingw* | *-*-msys* | *-*-pw32* | *-*-os2*)
      # By default, build only static.
      if test -z "$enable_shared"; then
        enable_shared=no
@@ -3795,7 +3795,7 @@
           case $host in
             *-*-darwin*)
               GMP_INCLUDE_MPN(x86_64/darwin.m4) ;;
-           *-*-mingw* | *-*-cygwin)
+           *-*-mingw* | *-*-cygwin | *-*-msys )
               GMP_INCLUDE_MPN(x86_64/dos64.m4) ;;
             *-openbsd*)
               GMP_DEFINE_RAW(["define(<OPENBSD>,1)"]) ;;

Thanks for the great GMP support.
Regards,
Ralph Peterson