build: don't warn when doxygen isn't found

Doxygen isn't so important that we need to warn when it is missing. I'd
assume it might even be missing more often than not for most builds.

Author: fanquake

configure.ac

@@ -111,9 +111,6 @@ AC_PATH_TOOL(READELF, readelf)
 AC_PATH_TOOL(CPPFILT, c++filt)
 AC_PATH_TOOL(OBJCOPY, objcopy)
 AC_PATH_PROG(DOXYGEN, doxygen)
-if test -z "$DOXYGEN"; then
-   AC_MSG_WARN([Doxygen not found])
-fi
 AM_CONDITIONAL([HAVE_DOXYGEN], [test -n "$DOXYGEN"])
 
 AC_ARG_VAR(PYTHONPATH, Augments the default search path for python module files)