Add Doxygen support

This change adds Doxygen support in the configure/make step. Doxygen is
optional, and the absence of Doxygen should not break the build.

Makefile.am

@@ -9,4 +9,17 @@ ACLOCAL_AMFLAGS = -I m4
 SUBDIRS = lib utils tests
 
 # Extra files that need to be in the distribution
-EXTRA_DIST = AUTHORS ChangeLog.md README.md LICENSE
+EXTRA_DIST = AUTHORS ChangeLog.md README.md LICENSE Doxyfile.in
+
+# Doxygen support
+if HAVE_DOXYGEN
+docs/.stamp:
+	$(AM_V_GEN)$(DOXYGEN) Doxyfile
+	$(AM_V_at)touch $@
+
+CLEANFILES = docs/.stamp
+
+all-local: docs/.stamp
+clean-local:
+	rm -rf $(top_builddir)/docs
+endif

configure.ac

@@ -27,9 +27,13 @@ AC_SUBST([X52_INCLUDE], ["-I \$(top_srcdir)/lib/libx52"])
 
 # Check for pthreads
 ACX_PTHREAD
-# LIBS="$PTHREAD_LIBS $LIBS"
-# CFLAGS="$PTHREAD_CFLAGS $CFLAGS"
-# CC="$PTHREAD_CC"
+
+# Doxygen Support
+AC_CHECK_PROGS([DOXYGEN], [doxygen])
+AM_CONDITIONAL([HAVE_DOXYGEN], [test -n "$DOXYGEN"])
+AM_COND_IF([HAVE_DOXYGEN],
+    [AC_CONFIG_FILES([Doxyfile])],
+    [AC_MSG_WARN(["Doxygen not found; continuing without doxygen support"])])
 
 AC_CONFIG_FILES([
     Makefile