pgsql: Provide DLLEXPORT markers for C functions via PG_FUNCTION_INFO_V

Provide DLLEXPORT markers for C functions via PG_FUNCTION_INFO_V1 macro.

Second try at the change originally made in commit 8518583cd;
this time with contrib updates so that manual extern declarations
are also marked with PGDLLEXPORT. The release notes should point
this out as a significant source-code change for extension authors,
since they'll have to make similar additions to avoid trouble on Windows.

Laurenz Albe, doc change by me

Patch: <A737B7A37273E048B164557ADEF4A58B53962ED8(at)ntex2010a(dot)host(dot)magwien(dot)gv(dot)at>

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/c8ead2a3974d3eada145a0e18940150039493cc9

Modified Files
--------------
contrib/hstore/hstore.h | 2 +-
contrib/ltree/ltree.h | 40 ++++++++++++++++++++--------------------
doc/src/sgml/xfunc.sgml | 17 +++++++++++++++++
src/include/fmgr.h | 7 +++----
4 files changed, 41 insertions(+), 25 deletions(-)