Do not link to sqlite3 on macos

On macos (darwin), sqlite3 is embedded using "the amalgamation". There
is therefore no need to link to the system sqlite3, and doing so can
cause unexpected behaviour/failures.

Also be explicit about darwin vs. non-darwin port_specs sources.
rebar3 seems to get confused when compiling under eunit command sometimes

c_src/sqlite3_drv.c

@@ -400,7 +400,7 @@ static int changes(sqlite3_drv_t *drv, char *buf, int len) {
 }
 
 static int enable_load_extension(sqlite3_drv_t* drv, char *buf, int len) {
-#ifndef ERLANG_SQLITE3_NO_LOAD_EXTENSION
+#ifdef ERLANG_SQLITE3_LOAD_EXTENSION
   char enable = buf[0];
   int result = sqlite3_enable_load_extension(drv->db, (int) enable);
   if (result) {

c_src/sqlite3_drv.h

@@ -4,6 +4,7 @@
 #endif
 
 #define _CRT_SECURE_NO_WARNINGS // secure functions aren't cross-platform
+#define ERLANG_SQLITE3_LOAD_EXTENSION // comment out if SQLite is built with SQLITE_OMIT_LOAD_EXTENSION
 
 #include <erl_driver.h>
 #include <ei.h>

rebar.config

@@ -1,10 +1,9 @@
 % -*- mode: erlang -*-
-{port_specs, [{"priv/sqlite3_drv.so", ["c_src/*.c"]},
+{port_specs, [{"^((?!darwin).)*$", "priv/sqlite3_drv.so", ["c_src/*.c"]},
               {"darwin", "priv/sqlite3_drv.so", ["c_src/*.c", "sqlite3_amalgamation/*.c"]}]}.
 {port_env, [{"DRV_CFLAGS", "$DRV_CFLAGS -Wall -Wextra -Wno-unused-parameter -Wstrict-prototypes"},
-            {"DRV_LDFLAGS", "$DRV_LDFLAGS -lsqlite3"},
+            {"^((?!darwin|win32).)*$", "DRV_LDFLAGS", "$DRV_LDFLAGS -lsqlite3"},
             {"ERL_LDFLAGS", " -L$ERL_EI_LIBDIR -lei"},
-            {".*darwin.*", "DRV_CFLAGS", "$DRV_CFLAGS -DERLANG_SQLITE3_NO_LOAD_EXTENSION"},
             {".*win32.*", "DRV_CFLAGS", "$DRV_CFLAGS /O2 /Isqlite3_amalgamation /Ic_src /W4 /wd4100 /wd4204 /wd4820 /wd4255 /wd4668 /wd4710 /wd4711"},
             {".*win32.*", "DRV_LDFLAGS", "$DRV_LDFLAGS sqlite3.lib"}]}.
 {cover_enabled, true}.