Update ax_boost_filesystem.m4 to serial 26

- Fixed cut-and-paste error.
- See http://savannah.gnu.org/patch/?7576 for further details.
- Use 'ls -r' rather than non-portable tool 'tac'.
- Simplify filepattern in ls expression so dylib (on OSX) are also detected.

src/m4/ax_boost_filesystem.m4

@@ -31,7 +31,7 @@
 #   and this notice are preserved. This file is offered as-is, without any
 #   warranty.
 
-#serial 21
+#serial 26
 
 AC_DEFUN([AX_BOOST_FILESYSTEM],
 [
@@ -81,14 +81,14 @@ AC_DEFUN([AX_BOOST_FILESYSTEM],
 			AC_DEFINE(HAVE_BOOST_FILESYSTEM,,[define if the Boost::Filesystem library is available])
             BOOSTLIBDIR=`echo $BOOST_LDFLAGS | sed -e 's/@<:@^\/@:>@*//'`
             if test "x$ax_boost_user_filesystem_lib" = "x"; then
-                for libextension in `ls $BOOSTLIBDIR/libboost_filesystem*.so* $BOOSTLIBDIR/libboost_filesystem*.dylib* $BOOSTLIBDIR/libboost_filesystem*.a* 2>/dev/null | sed 's,.*/,,' | sed -e 's;^lib\(boost_filesystem.*\)\.so.*$;\1;' -e 's;^lib\(boost_filesystem.*\)\.a*$;\1;' -e 's;^lib\(boost_filesystem.*\)\.dylib$;\1;'` ; do
+                for libextension in `ls -r $BOOSTLIBDIR/libboost_filesystem* 2>/dev/null | sed 's,.*/lib,,' | sed 's,\..*,,'` ; do
                      ax_lib=${libextension}
 				    AC_CHECK_LIB($ax_lib, exit,
                                  [BOOST_FILESYSTEM_LIB="-l$ax_lib"; AC_SUBST(BOOST_FILESYSTEM_LIB) link_filesystem="yes"; break],
                                  [link_filesystem="no"])
 				done
-                if test "x$link_program_options" != "xyes"; then
-                for libextension in `ls $BOOSTLIBDIR/boost_filesystem*.{dll,a}* 2>/dev/null | sed 's,.*/,,' | sed -e 's;^\(boost_filesystem.*\)\.dll.*$;\1;' -e 's;^\(boost_filesystem.*\)\.a*$;\1;'` ; do
+                if test "x$link_filesystem" != "xyes"; then
+                for libextension in `ls -r $BOOSTLIBDIR/boost_filesystem* 2>/dev/null | sed 's,.*/,,' | sed -e 's,\..*,,'` ; do
                      ax_lib=${libextension}
 				    AC_CHECK_LIB($ax_lib, exit,
                                  [BOOST_FILESYSTEM_LIB="-l$ax_lib"; AC_SUBST(BOOST_FILESYSTEM_LIB) link_filesystem="yes"; break],