fix compile error when both pnetcdf.h and netcdf.h are included in a C program

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1679 c368ceb8-fd28-0410-a457-e46ee1d7cd4a
add note on make ptest

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1680 c368ceb8-fd28-0410-a457-e46ee1d7cd4a