add --disable-doxygen-docs options (sf patch #1365935...

add --disable-doxygen-docs options (sf patch #1365935 https://sourceforge.net/tracker/index.php?func=detail&aid=1365935&group_id=13478&atid=313478)

configure.in

@@ -168,7 +168,10 @@ dnl check for i18n(internationalization); these are from libiconv/gettext
 AM_ICONV
 AM_LANGINFO_CODESET
 
-AC_CHECK_PROGS(DOXYGEN, doxygen)
+AC_ARG_ENABLE(doxygen_docs, AC_HELP_STRING([--disable-doxygen-docs], [Disable API documentation building via Doxygen]))
+if "x$enable_doxygen_docs" != "xno"; then
+	AC_CHECK_PROGS(DOXYGEN, doxygen)
+fi
 AM_CONDITIONAL(FLaC__HAS_DOXYGEN, test -n "$DOXYGEN")
 if test -n "$DOXYGEN" ; then
 AC_DEFINE(FLAC__HAS_DOXYGEN)