Configuration failed because libpq was not found.

Apologies @jeroen and @krlmlr for bringing this up as it seems to have been addressed previously: #118 and #350

I am running RHEL 8.9 using R 4.3.3 and attempted to install RPostgres, which failed with the following error:

trying URL 'https://cloud.r-project.org/src/contrib/RPostgres_1.4.7.tar.gz'
Content type 'application/x-gzip' length 1476313 bytes (1.4 MB)
==================================================
downloaded 1.4 MB

* installing *source* package ‘RPostgres’ ...
** package ‘RPostgres’ successfully unpacked and MD5 sums checked
** using staged installation
Using PKG_CFLAGS=
Using PKG_LIBS=-lpq
Using PKG_PLOGR=
------------------------- ANTICONF ERROR ---------------------------
Configuration failed because libpq was not found. Try installing:
 * deb: libpq-dev libssl-dev (Debian, Ubuntu, etc)
 * rpm: postgresql-devel (Fedora, EPEL)
 * rpm: postgreql8-devel, postgresql92-devel, postgresql93-devel, or postgresql94-devel (Amazon Linux)
 * csw: postgresql_dev (Solaris)
 * brew: libpq (OSX)
If libpq is already installed, check that either:
(i)  'pkg-config' is in your PATH AND PKG_CONFIG_PATH contains
     a libpq.pc file; or
(ii) 'pg_config' is in your PATH.
If neither can detect , you can set INCLUDE_DIR
and LIB_DIR manually via:
R CMD INSTALL --configure-vars='INCLUDE_DIR=... LIB_DIR=...'
--------------------------[ ERROR MESSAGE ]----------------------------
<stdin>:1:10: fatal error: libpq-fe.h: No such file or directory
compilation terminated.
-----------------------------------------------------------------------
ERROR: configuration failed for package ‘RPostgres’
* removing ‘/opt/R/4.3.3/lib/R/library/RPostgres’

The downloaded source packages are in
	‘/tmp/RtmpFVocGF/downloaded_packages’
Updating HTML index of packages in '.Library'
Making 'packages.html' ... done
Warning message:
In install.packages("RPostgres") :
  installation of package ‘RPostgres’ had non-zero exit status

I attempted the following:

1. sudo yum install postgresql-devel
2. sudo yum install libpq
   which did install these things, but did not resolve the install error.

I tried export PATH=$PATH:/usr/pgsql-15/bin/pg_config as well (thinking this would address (ii) above, by adding pg_config to my PATH) but this also did not resolve the error. I can confirm that neither condition in (i) is true, nor do I know how to address them.

Other output requested in the referenced issues:

$ pkg-config --cflags libpq
Package libpq was not found in the pkg-config search path.
Perhaps you should add the directory containing `libpq.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libpq', required by 'virtual:world', not found

... and the closest I can find to libpq.pc appears to be /usr/lib64/libpq.so

No source files or tests are named. Reproduce the RPostgres installation on RHEL 8.9 and inspect the package configuration checks around pkg-config, pg_config, and libpq headers; done means the documented PostgreSQL client setup is detected and installation succeeds.