modification to configure.ac: set darwin_ver correctly in configure

[thisisgk]

Hi,
I tried to build libcaca from git master on macOS. When I ran ./bootstrap then ./configure, I noticed this output:

./configure: line 19422: [: : integer expression expected

This is because darwin_ver is not set. Lines 19421-19422 in generated configure:

    darwin_ver="$(echo "${target_os}" | sed -ne 's/^0-9*\(0-9\+\).*/\1/p')"
    if [ "$darwin_ver" -lt "13" ]; then

from configure.ac:

libcaca/configure.ac

Lines 299 to 300 in f42aa68

	darwin_ver="$(echo "${target_os}" | sed -ne 's/[^0-9]*\([0-9]\+\).*/\1/p')"
	if [[ "$darwin_ver" -lt "13" ]]; then

As can be seen, brackets are removed from sed command. And, actually, \+ does nothing in macOS sed (came from BSD sed). [0-9]\+ should be written as [0-9][0-9]*.
Further, since test command is mainly used in configure.ac, I think it would be better to write if test here too.

So, I'd like to propose this modification to configure.ac:

--- a/configure.ac
+++ b/configure.ac
@@ -296,8 +296,8 @@ if test "${enable_cocoa}" != "no"; then
     esac
     CC="${CC:-gcc-${GCC_VERSION}}"
     CXX="${CXX:-g++-${GCC_VERSION}}"
-    darwin_ver="$(echo "${target_os}" | sed -ne 's/[^0-9]*\([0-9]\+\).*/\1/p')"
-    if [[ "$darwin_ver" -lt "13" ]]; then
+    darwin_ver="$(echo "${target_os}" | sed -ne 's/[[^0-9]]*\([[0-9]][[0-9]]*\).*/\1/p')"
+    if test "$darwin_ver" -lt "13"; then
       MACOSX_SDK_FRAMEWORKS="${MACOSX_SDK_FRAMEWORKS:--F${MACOSX_SDK}/System/Library/Frameworks}"
       CPPFLAGS="${CPPFLAGS} ${ARCH} ${MACOSX_SDK_FRAMEWORKS}"
       CFLAGS="${CFLAGS} ${MACOSX_SDK_CFLAGS} -mmacosx-version-min=10.5"