configure: Do not add extralibs to avresample .pc file

The library does not have external dependencies.
build: Support executable only ldflags

The options is useful to build position-independent executables on
hardened systems (e.g. Android L and Gentoo Hardened).
http: Provide an option to override the HTTP method

Certain servers accept only PUT as valid method.
http: Do move the class instantiation in the conditional block

Remove a warning if https support is disabled.