to get TOPSRCDIR that avoids use of
TEST_ENVIRONMENT and makes automake and cmake
more consistent.
Basic assumption is that abs_top_srcdir (and
cmake equivalent) is known at 'make check' time,
so we can use -D flag to compile a program that
has the value of abs_top_srcdir embedded into it
as a constant.
We define two new files in ncdap_test:
1. t_srcdir.h -- provide a gettopsrcdir() function
to return the topsrcdir value to the test program.
2. topsrcdir.c -- a program that calls gettopsrcdir()
and prints its output (minus any newline) on stdout.
This is used in .sh files to get topsrcdir.
