Fix cross compilation

Fixes Issue 106 (#106)

modified:   external/dinglibs.m4
A warning message is displayed to the user to make sure lib_ini_config
supprts extenden INI_MS_DETECT, since the test cannot be run in a cross
compilation scenario.
modified:   external/docbook.m4
Autoconf Macro AC_CHECK_FILE has been replaced by an explicit if ! test
-e ..., as AC_CHECK_FILE is not available in cross compilation scenario.

external/dinglibs.m4

@@ -142,6 +142,6 @@ cleanup:
     return ret;
 }
 ]])]
-,, [AC_MSG_ERROR(["ini_config library must support extended INI_MS_DETECT. See: https://pagure.io/SSSD/ding-libs/pull-request/3172"])])
+,, [AC_MSG_ERROR(["ini_config library must support extended INI_MS_DETECT."])], AC_MSG_WARN(["Cross Compiling. Make sure your ini_config library supports extended INI_MS_DETECT"]))
 
 ])

external/docbook.m4

@@ -23,7 +23,7 @@ dnl Checks if the XML catalog given by FILE exists and
 dnl if a particular URI appears in the XML catalog
 AC_DEFUN([CHECK_STYLESHEET],
 [
-  AC_CHECK_FILE($1, [], [AC_MSG_ERROR([could not find XML catalog])])
+  if ! test -e $1; then AC_MSG_ERROR([could not find XML catalog]); fi
 
   AC_MSG_CHECKING([for ifelse([$3],,[$2],[$3]) in XML catalog])
   if AC_RUN_LOG([$XMLCATALOG --noout "$1" "$2" >&2]); then