Errors while installing PostGIS by an unusual method

I am trying to install PostGIS on a RHEL_5 system and failing at it
miserably. The way I am trying to install it is as follows. I copy over all
the artifacts of postgresql server as in the bin, lib, share directories of
postgresql server in a directory called pgql and I place it in the root
directory i.e /pgsql is the directory which contains things like pg_config
and all the other libs and bins that one gets by installing postgresql
using the standard installation. All the dependencies of PostGIS get
installed successfully using the standard build process of *./configure
--prefix=/pgsql, make and make install* but when I issue the following
command for building PostGIS using the same process :

./configure --with-gdalconfig=/pgsql/bin/gdal-config
--with-geosconfig=/pgsql/bin/geos-config --with-projdir=/pgsql
--with-jsondir=/pgsql --with-pgconfig=/pgsql/bin/pg_config

I get the following error in the configure step :

checking PostgreSQL version... PostgreSQL 9.6

checking libpq-fe.h usability... no

checking libpq-fe.h presence... no

checking for libpq-fe.h... no

configure: error: could not find libpq-fe.h"

to remedy this error I tried placing the include files in the include
directory of postgresql by find the appropriate path using

/pgsql/bin/pg_config --includedir

and then when I try to install PostGIS, it still fails. Can anyone suggest
some workaround to build PostGIS using this non-standard approach of
building PostGIS?

Edit: When I try to add all the include files which includes the libpq-fe.h
and libpq headers as well along with other header I get the following error
when configuring PostGIS

checking for PQserverVersion in -lpq... no

configure: error: could not find libpq