build: enforce minimum required Windows version (7)

Instruct the linker to set the major & minor subsystem versions in the PE
header to 6 & 1 (NT 6.1 which corresponds to Windows 7). Similar to
macOS, the binary will now refuse to run on unsupported versions of
Windows.

Author: fanquake

configure.ac

@@ -595,6 +595,8 @@ case $host in
      archive_cmds_CXX="\$CC -shared \$libobjs \$deplibs \$compiler_flags -static -o \$output_objdir/\$soname \${wl}--enable-auto-image-base -Xlinker --out-implib -Xlinker \$lib"
      postdeps_CXX=
 
+     dnl We require Windows 7 (NT 6.1) or later
+     AX_CHECK_LINK_FLAG([[-Wl,--major-subsystem-version -Wl,6 -Wl,--minor-subsystem-version -Wl,1]],[LDFLAGS="$LDFLAGS -Wl,--major-subsystem-version -Wl,6 -Wl,--minor-subsystem-version -Wl,1"],,[[$LDFLAG_WERROR]])
      ;;
   *darwin*)
      TARGET_OS=darwin