Remove -Wdeclaration-after-statement from PG_CFLAGS

[svenklemm]

Postgres CFLAGS includes -Wdeclaration-after-statement which leads
to problems when compiling with -Werror since we aim for C99 and allow
that so we strip this flag from PG_CFLAGS before adding postgres flags
to our own

[mkindahl]

Looks fine, but I think you can remove the space before the flag.

[codecov]

Codecov Report

Merging #1637 into master will increase coverage by 0.34%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1637      +/-   ##
==========================================
+ Coverage   91.54%   91.89%   +0.34%     
==========================================
  Files         144      142       -2     
  Lines       21959    21014     -945     
==========================================
- Hits        20102    19310     -792     
+ Misses       1857     1704     -153

Flag	Coverage Δ
#cron	?
#pr	91.89% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/plan_add_hashagg.c	43.33% <0%> (-45.01%)	⬇️
src/telemetry/uuid.c	84.61% <0%> (-15.39%)	⬇️
src/planner_import.c	58.65% <0%> (-10.95%)	⬇️
src/cache_invalidate.c	78.12% <0%> (-2.44%)	⬇️
src/bgw_policy/chunk_stats.c	83.92% <0%> (-1.56%)	⬇️
src/loader/loader.c	91.57% <0%> (-1.25%)	⬇️
src/histogram.c	89.02% <0%> (-1.22%)	⬇️
src/estimate.c	86.17% <0%> (-0.83%)	⬇️
src/telemetry/telemetry.c	79.55% <0%> (-0.71%)	⬇️
src/cache.c	84.82% <0%> (-0.65%)	⬇️
... and 76 more

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