Prepare for GCC 14

[jamesjer]

GCC 14 is going to be pickier about things like implicit return values, mismatched pointer types, and implicit conversions between pointers and ints. The Fedora project is trying to prepare for GCC 14:

• https://fedoraproject.org/wiki/Changes/PortingToModernC
• https://fedoraproject.org/wiki/Toolchain/PortingToModernC

This commit fixes several code issues that will be errors when GCC 14 is released:

• bgl_backtrace_foreach has a return type of void, but contains a return statement with a value
• bgl_get_session_callback has the wrong type, due to a missing const keyword on one parameter
• bgl_ssl_connection_get_peer_certificate contains an assigment of one pointer type to a variable of a different pointer type
• autoconf/openssl-getter fails because the test code returns a pointer type from main, which is declared to return int

[manuel-serrano]

Thank you so much. Your contribution helps.