[PATCH 1/2] use AC_SEARCH_LIBS instead of AC_CHECK_LIBS for m, pthread and ltdl

[Andrei Kholodnyi]

> Would you have a problem with me updating the commit message on this
> commit? ?There are actually two changes going on here. ?One is the
> switch from AC_CHECK_LIB to AC_SEARCH_LIBS and the other is the switch
> from libdl to libltdl.

ah, I missed it, it was a copy/paste from opentts.
yes, please add it

Does anyone know why replacing
-AC_CHECK_LIB(m,
-        sqrt, echo "",
-        echo "*** Required math library missing! See INSTALL .";exit 1)

with

+AC_SEARCH_LIBS([sqrt], [m], [],
+       [AC_MSG_FAILURE([Math library missing])])

allows proper ttspico lib discovery:
        [AC_CHECK_LIB([ttspico],
                [pico_initialize],
                [with_pico=yes],
                [AS_IF([test $with_pico = "yes"],
                        [AC_MSG_FAILURE([pico SVOX is not available])])])])

w/o this change ttspico is not discovered.