configure: Split out msvc as a separate target OS

The name mingw32 as target OS is both misleading, and very little
of the target OS specific settings actually match.

Since the target OS default is set based on uname, the default
(which on MSYS is set to mingw) is overridden by --toolchain=msvc.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -1989,8 +1989,6 @@ if enabled cross_compile; then
         die "Must specify target arch and OS when cross-compiling"
 fi
 
-set_default arch target_os
-
 ar_default="${cross_prefix}${ar_default}"
 cc_default="${cross_prefix}${cc_default}"
 nm_default="${cross_prefix}${nm_default}"
@@ -2015,13 +2013,14 @@ case "$toolchain" in
         ld_default="c99wrap link"
         nm_default="dumpbin -symbols"
         ar_default="lib"
+        target_os_default="win32"
     ;;
     ?*)
         die "Unknown toolchain $toolchain"
     ;;
 esac
 
-set_default cc pkg_config sysinclude
+set_default arch cc pkg_config sysinclude target_os
 enabled cross_compile || host_cc_default=$cc
 set_default host_cc
 
@@ -2032,7 +2031,7 @@ fi
 
 exesuf() {
     case $1 in
-        mingw32*|cygwin*|*-dos|freedos|opendos|os/2*|symbian) echo .exe ;;
+        mingw32*|win32|win64|cygwin*|*-dos|freedos|opendos|os/2*|symbian) echo .exe ;;
     esac
 }
 
@@ -2761,6 +2760,11 @@ case $target_os in
         enable dos_paths
         add_cppflags -U__STRICT_ANSI__
         ;;
+    win32|win64)
+        objformat="win32"
+        ranlib=:
+        enable dos_paths
+        ;;
     cygwin*)
         target_os=cygwin
         shlibdir_default="$bindir_default"