pgsql: Replace internal C function pg_hypot() by standard hypot()

Replace internal C function pg_hypot() by standard hypot()

The code comment said, "It is expected that this routine will
eventually be replaced with the C99 hypot() function.", so let's do
that now.

This function is tested via the geometry regression test, so if it is
faulty on any platform, it will show up there.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/170308e6-a7a3-4484-87b2-f960bb564afa%40eisentraut.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/8fe4aef8297b9f6b27a8f9422f3fdef1f79bb189

Modified Files
--------------
src/backend/access/spgist/spgproc.c |  2 +-
src/backend/utils/adt/geo_ops.c     | 76 ++-----------------------------------
src/backend/utils/adt/geo_spgist.c  |  2 +-
src/include/utils/geo_decls.h       |  8 ----
4 files changed, 6 insertions(+), 82 deletions(-)