about pgenv.h file

May platform is SUN workstation , SUN OS 5.5.1

I want install v6.4.  When I type "gmake all", it have error.

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% ./configure --with-template=solaris_sparc_gcc
--with-CXX=/usr/local/bin/g++
% gmake all
***************
gmake -C libpq++ all
In file include from pgenv.cc
pgenv.h:21: string: No such file or directory
-----------------------

I look the ../src/interfaces/libpq++/pgenv.h 21 line
#include 
In my system there is no "string" file.
How to solve it?

Thank you very much!