Adhere to GNU Makefile conventions

[ripatel-fd]

Another low prio issue --

Most projects using GNU Make adhere to the following conventions: https://www.gnu.org/prep/standards/html_node/Makefile-Conventions.html

We are mostly there, with the exception of

• an install target with DESTDIR support (https://www.gnu.org/prep/standards/html_node/Standard-Targets.html)
• an include dir install target
• a pkg-config pc file and library install targets (debatable, as using Firedancer as a library is not officially supported, and breaking ABI changes are expected)