feat: Build against libpq 15

Signed-off-by: Wolfgang Walther <walther@technowledgy.de>
Co-authored-by: Wolfgang Walther <walther@technowledgy.de>

nix/patches/postgresql-atexit.patch

@@ -0,0 +1,11 @@
+--- a/src/interfaces/libpq/Makefile
++++ b/src/interfaces/libpq/Makefile
+@@ -118,7 +118,7 @@ backend_src = $(top_srcdir)/src/backend
+ libpq-refs-stamp: $(shlib)
+ ifneq ($(enable_coverage), yes)
+ ifeq (,$(filter aix solaris,$(PORTNAME)))
+-	@if nm -A -u $< 2>/dev/null | grep -v __cxa_atexit | grep exit; then \
++	@if nm -A -u $< 2>/dev/null | grep " exit"; then \
+ 		echo 'libpq must not be calling any function which invokes exit'; exit 1; \
+ 	fi
+ endif