Handle disable doxygen configure option

Previously this broke when using the --disable-doxygen-doc option


Former-commit-id: f89fa64e4b00c94f5d83e10e6983e6708c99f6ad

configure.ac

@@ -290,8 +290,9 @@ AC_MSG_RESULT($rtc_ok)
 AC_PATH_PROG([GIT_CMD], [git])
 AC_SUBST([GIT_CMD])
 
-# Make the documentation.
+# Make the documentation. Add conditional to handle disable option.
 DX_INIT_DOXYGEN(libswift,doc/Doxyfile,doc/)
+AM_CONDITIONAL([HAVE_DOXYGEN], [test "$ac_cv_path_ac_pt_DX_DOXYGEN" != ""])
 
 # Handle .in files.
 AC_CONFIG_FILES([Makefile src/Makefile examples/Makefile doc/Makefile doc/Doxyfile])

doc/Makefile.am

+
 doxyfile.stamp: 
+if HAVE_DOXYGEN
 	@DX_ENV@ @DX_DOXYGEN@ Doxyfile
 	echo `date` > doxyfile.stamp
+endif
 
 CLEANFILES = doxyfile.stamp