Don't hardcode -ldl, use proper test.
Don't hardcode -ldl, use proper test.

AC_CHECK_HEADERS -> AC_CHECK_HEADER when there is only one header;

Allow modules support and modules themselves to be compiled only
when system supports dlopen et all.

sippy/rtpproxy,jevonearth/rtpproxy,sippy/rtpproxy,dsanders11/rtpproxy,synety-jdebp/rtpproxy,dsanders11/rtpproxy,jevonearth/rtpproxy,synety-jdebp/rtpproxy,synety-jdebp/rtpproxy,jevonearth/rtpproxy,dsanders11/rtpproxy,jevonearth/rtpproxy,synety-jdebp/rtpproxy,sippy/rtpproxy