automake: Check for 'indent' and fall back to 'cat' if not found.

The glapi generator code uses indent to produce more readable code.
However, we don't want to make GNU indent a hard build dependency; check
for it in configure.ac and fall back to 'cat' if it's not available.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50484
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Acked-by: Ben Widawsky <ben@bwidawsk.net>
diff --git a/configure.ac b/configure.ac
index 20bd00b..9fb8149 100644
--- a/configure.ac
+++ b/configure.ac
@@ -72,6 +72,11 @@
 
 AC_PATH_PROG([PERL], [perl])
 
+AC_CHECK_PROG(INDENT, indent, indent, cat)
+if test "x$INDENT" != "xcat"; then
+    AC_SUBST(INDENT_FLAGS, '-i4 -nut -br -brs -npcs -ce -T GLubyte -T GLbyte -T Bool')
+fi
+
 dnl Our fallback install-sh is a symlink to minstall. Use the existing
 dnl configuration in that case.
 AC_PROG_INSTALL