configure: Use pg_config to locate the library location too

Also, allow the pg_config binary to be specified through the PG_CONFIG
environment variable, defaulting to 'pg_config' if unset. Explicitly
setting PG_CONFIG to an empty string will forcibly disable PostgreSQL
support, even if a PostgreSQL library is installed.

Changelog-Fixed: build: On systems with multiple installed versions of the PostgreSQL client library, C-Lightning might link against the wrong version or fail to find the library entirely. `./configure` now uses `pg_config` to locate the library.

Makefile

@@ -228,7 +228,7 @@ endif
 
 # If we have the postgres client library we need to link against it as well
 ifeq ($(HAVE_POSTGRES),1)
-LDLIBS += -lpq
+LDLIBS += $(POSTGRES_LDLIBS)
 endif
 
 default: show-flags all-programs all-test-programs doc-all

configure

@@ -230,8 +230,10 @@ fi
 require 'python3-mako' "You need the mako module for python3: see doc/INSTALL.md" python3 -c 'import mako'
 
 POSTGRES_INCLUDE=""
-if command -v pg_config 2> /dev/null; then
-    POSTGRES_INCLUDE="-I$(pg_config --includedir)"
+POSTGRES_LDLIBS=""
+if command -v "${PG_CONFIG-pg_config}" >/dev/null; then
+    POSTGRES_INCLUDE="-I$("${PG_CONFIG-pg_config}" --includedir)"
+    POSTGRES_LDLIBS="-L$("${PG_CONFIG-pg_config}" --libdir) -lpq"
 fi
 
 rm -f $CONFIG_VAR_FILE.$$
@@ -290,7 +292,7 @@ int main(void)
 var=HAVE_POSTGRES
 desc=postgres
 style=DEFINES_EVERYTHING|EXECUTE|MAY_NOT_COMPILE
-link=-lpq
+link=$POSTGRES_LDLIBS
 code=
 #include <libpq-fe.h>
 #include <stdio.h>
@@ -360,6 +362,7 @@ add_var CWARNFLAGS "$CWARNFLAGS"
 add_var CDEBUGFLAGS "$CDEBUGFLAGS"
 add_var COPTFLAGS "$COPTFLAGS"
 add_var POSTGRES_INCLUDE "$POSTGRES_INCLUDE"
+add_var POSTGRES_LDLIBS "$POSTGRES_LDLIBS"
 add_var VALGRIND "$VALGRIND"
 add_var DEVELOPER "$DEVELOPER" $CONFIG_HEADER
 add_var EXPERIMENTAL_FEATURES "$EXPERIMENTAL_FEATURES" $CONFIG_HEADER