Don't use MinGW strtod workaround when building for MinGW-w64.

The host triplet for MinGW-w64 has changed to more closely match that
for MinGW, so we need to update the pattern in the test.

aclocal.m4

@@ -225,7 +225,7 @@ AC_DEFUN([AX_C99_STRTOD],
 [# On MinGW we need to jump through hoops to get a C99 compliant strtod().
  # mingw-w64 doesn't need this, and the 64-bit version doesn't support it.
 case "${host}" in
-    *-*-mingw32)
+    *-pc-mingw32)
         LDFLAGS+=" -Wl,--undefined=___strtod,--wrap,strtod,--defsym,___wrap_strtod=___strtod"
         ;;
 esac