community/libmicrohttpd: upgrade to 0.9.75 and some optimisations

This MR do the following:

• libmicrohttpd upgraded to the latest upstream version 0.9.75,
• removed rebuild of configure and autotools files,
• removed installation of useless private header,
• removed unused dependencies,
• curl-dev moved to checkdepends as curl is used only for interoperability testing,
• added configure setting to minimise binary size.

Every change is placed in the individual commit to simplify review.

The whole MR could be squashed into single commit.