libpq++ installation error

I am trying to install and compile libpq++ i get this error

configure: error:
Can't find libpq-fe.h in . Are you sure the libpq
headers are installed correctly? They should be in the directory returned by
"pg_config --includedir".
If you do have libpq (the C-language client library for PostgreSQL) installed,
make sure you have the related development materials--mainly its header files--
as well as the library binary. Some system distributions keep the two in
seperate packages with names like "alibrary" and "alibrary-dev", respectively.
In that case, make sure you have the latter installed as well.

i have cheacked pg_config --includedir it gave me a correct path also i checked the include directory i found the requsted header file there

regards

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