pgsql: Re-include pqsignal() in libpq.

Re-include pqsignal() in libpq.

We need this in non-ENABLE_THREAD_SAFETY builds, and also to satisfy
the exports.txt entry; while it might be a good idea to remove the
latter, I'm hesitant to do so except in the context of an intentional
ABI break.  At least we don't have a separately maintained source file
for it anymore.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b1fae823ee46a26e7e557591d659351835742537

Modified Files
--------------
src/interfaces/libpq/.gitignore |    1 +
src/interfaces/libpq/Makefile   |   15 ++++++++-------
src/interfaces/libpq/bcc32.mak  |    7 +++++++
src/interfaces/libpq/win32.mak  |    7 +++++++
4 files changed, 23 insertions(+), 7 deletions(-)