pgsql: Fix compiler warnings around _CRT_glob

Fix compiler warnings around _CRT_glob

Newer compilers warned about

extern int _CRT_glob = 0;

which is indeed a mysterious C construction, as it combines "extern"
and an initialization. It turns out that according to the C standard,
the "extern" is ignored here, so we can remove it to resolve the
warnings. But then we also need to add a real extern
declaration (without initializer) to satisfy
-Wmissing-variable-declarations.

(Note that this code is only active on MinGW.)

Discussion: /messages/by-id/1053279b-da01-4eb4-b7a3-da6b5d8f73d1@eisentraut.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3e908fb54ff8fa857180bc212ca79c7bd95dcc2c

Modified Files
--------------
src/common/exec.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)