Merge #19301: build: don't warn when doxygen isn't found

fa84edb93c build: don't warn when doxygen isn't found (fanquake)

Pull request description:

  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.

ACKs for top commit:
  MarcoFalke:
    Fine with me ACK fa84edb93c
  hebasto:
    ACK fa84edb93c, I have reviewed the code and it looks OK, I agree it can be merged.

Tree-SHA512: 793ebf01a8a5d48b78a70fdef0022633fca59b30074c960ebb21589e3bd98992b8304621a2d999195d12172ed30fe9eefeeb2a952d58853cf58e8d9902b0090c

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)