code coverage patch

I've attached a patch that allows the generation of code coverage
statistics. To test it, apply the patch, then:

autoconf
./configure --enable-coverage
make
make check (or execute any other application against the database to see
the coverage of that app)
make coverage
make coverage_out

You will find a coverage directory in your build directory. Open
coverage/index.html to see the code coverage.

There are a couple of issues.

gcov gets confused when source files are generated. I eliminated
src/backend/bootstrap and ../parser from coverage analysis to avoid
errors of this type. With a vpath build only, code coverage fails on
another generated .c file, src/backend/utils/misc/guc-file.c.

Also, two tests fail with the following diff when the build is
configured with --enable-coverage.
RETURNS trigger
AS
'/home/michelle/trunkClean/pgsql/src/test/regress/../../../contrib/spi/refint.so'
LANGUAGE C;
+ ERROR: could not load library
"/home/michelle/trunkClean/pgsql/src/test/regress/../../../contrib/spi/refint.so":
/home/michelle/trunkClean/pgsql/src/test/regress/../../../contrib/spi/refint.so:
undefined symbol: __gcov_merge_add
CREATE FUNCTION check_foreign_key ()
RETURNS trigger
AS
'/home/michelle/trunkClean/pgsql/src/test/regress/../../../contrib/spi/refint.so'
LANGUAGE C;
+ ERROR: could not load library
"/home/michelle/trunkClean/pgsql/src/test/regress/../../../contrib/spi/refint.so":
/home/michelle/trunkClean/pgsql/src/test/regress/../../../contrib/spi/refint.so:
undefined symbol: __gcov_merge_add
CREATE FUNCTION autoinc ()
RETURNS trigger
AS
'/home/michelle/trunkClean/pgsql/src/test/regress/../../../contrib/spi/autoinc.so'
LANGUAGE C;
+ ERROR: could not load library
"/home/michelle/trunkClean/pgsql/src/test/regress/../../../contrib/spi/autoinc.so":
/home/michelle/trunkClean/pgsql/src/test/regress/../../../contrib/spi/autoinc.so:
undefined symbol: __gcov_merge_add
CREATE FUNCTION funny_dup17 ()
RETURNS trigger
AS '/home/michelle/trunkClean/pgsql/src/test/regress/regress.so'

Please test and comment.

-- Michelle

--
Michelle Caisse Sun Microsystems
California, U.S. http://sun.com/postgresql