move .pc file generation from Makefile.am to configure.ac

This change moves the logic for generating pkg-config (.pc) files from
Makefile.am to configure.ac using AC_CONFIG_COMMANDS. This ensures that
these files are generated at configure time. The generation logic preserves
the "echo" approach to maintain control over the output content and
avoid standard template instantiation if it was desired to keep the script-like behavior.

Also updated Makefile.am to remove the generation rules and moved the .pc files
from CLEANFILES to DISTCLEANFILES so they are cleaned up by `make distclean`
rather than `make clean`.

[alkondratenko@gmail.com: made trivial comment update]
Signed-off-by: Aliaksei Kandratsenka <alkondratenko@gmail.com>