pgsql: Fix build failure on header generation with repetitive builds of

Fix build failure on header generation with repetitive builds of MSVC

GenerateConfigHeader() in Solution.pm was complaining about unused
define symbols even if a newer config header was not generated, causing
successive build attempts with MSVC to fail.

Oversight in commit 8f4fb4c.

Author: Kyotaro Horiguchi
Reviewed-by: Juan José Santamaría Flecha
Discussion: /messages/by-id/20200218.160500.44393633318853097.horikyota.ntt@gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/59f9cd9dd5e4db8c59c57a17388c17564a3211a3

Modified Files
--------------
src/tools/msvc/Solution.pm | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)