pgsql: Add support for gamma() and lgamma() functions.

Add support for gamma() and lgamma() functions.

These are useful general-purpose math functions which are included in
POSIX and C99, and are commonly included in other math libraries, so
expose them as SQL-callable functions.

Author: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Reviewed-by: Stepan Neretin <sncfmgg(at)gmail(dot)com>
Reviewed-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Dmitry Koval <d(dot)koval(at)postgrespro(dot)ru>
Reviewed-by: Alexandra Wang <alexandra(dot)wang(dot)oss(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAEZATCXpGyfjXCirFk9au+FvM0y2Ah+2-0WSJx7MO368ysNUPA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a3b6dfd410699a19209be3cb3c7a74e5ff1da830

Modified Files
--------------
doc/src/sgml/func.sgml | 38 ++++++++++++++++
src/backend/utils/adt/float.c | 88 ++++++++++++++++++++++++++++++++++++
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 7 +++
src/test/regress/expected/float8.out | 57 +++++++++++++++++++++++
src/test/regress/sql/float8.sql | 23 ++++++++++
6 files changed, 214 insertions(+), 1 deletion(-)