libpq is the C application programmer's interface to PostgreSQL. libpq
+ is a set of library functions that allow client programs to pass que
+ries to the PostgreSQL backend server and to receive the results of t
+hese queries.

libpq is also the underlying engine for several other PostgreSQL appli
+cation interfaces, including those written for C++, Perl, Python, Tcl
+ and ECPG. So some aspects of libpq's behavior will be important to y
+ou if you use one of those packages (from https://www.postgresql.org/
+docs/current/libpq.htm)
[download]

If you installed from source, then it may be that it does not install libpq by default although it's contained in the source, perhaps for doing some testing. Check the makefile targets, perhaps there is one to install libpq.

For installing Pg with your package manager, it could be that libpq is considered optional (this is what I sensed from cited doc) and therefore it is not bundled with the standard Pg package. In fedora there is a separate libpq package available. And you found the package for your system.