There is an updated pull request by sgn against master on the void-packages repository

https://github.com/sgn/void-packages postgresql-fix-pg-config
https://github.com/void-linux/void-packages/pull/20783

postgresql: fix pg_config --*flags
Current `pg_config` reports flags included `$SYSROOT`,

	$ pg_config --cppflags
	-D_GNU_SOURCE -I/usr/aarch64-linux-musl/usr/include/libxml2

Fix the build system to remove it from $CPPFLAGS $CFLAGS $LDFLAGS.

Also ship a new pg_config.sh for cross-compiling other packages that
depends on postgresql.
That script was mostly translated from C code.

(Partial?) fix for #19642 

A patch file from https://github.com/void-linux/void-packages/pull/20783.patch is attached