pgsql: Fix make build on MinGW

Fix make build on MinGW

Revert a couple of the simplifications done in commit 721856ff24b
because platforms without ln -s, where LN_S='cp -pR', such as MinGW,
required the specific previous incantations.

Reported-by: Noah Misch <noah@leadboat.com>
Discussion: https://www.postgresql.org/message-id/20240616193448.28@rfd.leadboat.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/58445651dbc6182e1ff4100f6428ba6a261407f9

Modified Files
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src/backend/Makefile | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)