tests require ${PostgreSQL_INCLUDE_DIRS}

With the latest master I can not build the tests anymore, due to errors:

FAILED: src/test/pq/CMakeFiles/basic_datatypes.dir/basic_datatypes.cpp.o 
/data/Tools/lib/ccache/c++   -I/home/user/Source/taopq/include -ggdb3 -fno-omit-frame-pointer -O1 -m64 -march=nehalem -mtune=haswell  -DDEBUG -g -fPIE   -pedantic -Wall -Wextra -Wshadow -Werror -std=c++1z -MD -MT src/test/pq/CMakeFiles/basic_datatypes.dir/basic_datatypes.cpp.o -MF src/test/pq/CMakeFiles/basic_datatypes.dir/basic_datatypes.cpp.o.d -o src/test/pq/CMakeFiles/basic_datatypes.dir/basic_datatypes.cpp.o -c /home/user/Source/taopq/src/test/pq/basic_datatypes.cpp
In file included from /home/user/Source/taopq/include/tao/pq/parameter_traits.hpp:7:0,
                 from /home/user/Source/taopq/include/tao/pq/transaction.hpp:14,
                 from /home/user/Source/taopq/include/tao/pq/connection.hpp:13,
                 from /home/user/Source/taopq/src/test/pq/basic_datatypes.cpp:11:
/home/user/Source/taopq/include/tao/pq/internal/parameter_binary_traits.hpp:16:10: fatal error: libpq-fe.h: No such file or directory
 #include <libpq-fe.h>
          ^~~~~~~~~~~~
compilation terminated.

I'm not certain what the best fix is, therefore no PR attached. Should the tests just add ${PostgreSQL_INCLUDE_DIRS}?

taocpp / taopq

tests require ${PostgreSQL_INCLUDE_DIRS} #21