Rework the indent target.

- Check indent is present at ./configure stage;
  - If gindent(1) is installed, use it instead of indent(1);

configure.ac

@@ -90,6 +90,16 @@ then
 fi
 AM_CONDITIONAL(DOC_ENABLED, [test x"$enable_doc" = xyes])
 
+# The indent target require GNU indent
+AC_PATH_PROG([INDENT], [indent])
+AC_PATH_PROG([GINDENT], [gindent])
+if test x"$GINDENT" != x""; then
+  INDENT="$GINDENT"
+fi
+
+AC_SUBST(INDENT)
+AM_CONDITIONAL(HAS_INDENT, [test x"$INDENT" != x""])
+
 # Dependencies
 PKG_CONFIG_REQUIRES=""