New option "--disable-system-torrc" to not read torrc from etc

Implements 13037.

changes/ticket13037

+  o Minor features (build):
+    - New --disable-system-torrc compile-time option to prevent Tor from
+      looking for a system-wide torrc or torrc-defaults tile. Resolves
+      ticket 13037.

configure.ac

@@ -45,6 +45,8 @@ AC_ARG_ENABLE(unittests,
    AS_HELP_STRING(--disable-unittests, [Don't build unit tests for Tor. Risky!]))
 AC_ARG_ENABLE(coverage,
    AS_HELP_STRING(--enable-coverage, [Enable coverage support in the unit-test build]))
+AC_ARG_ENABLE(system-torrc,
+   AS_HELP_STRING(--disable-system-torrc, [Don't look for a system-wide torrc file]))
 
 AM_CONDITIONAL(UNITTESTS_ENABLED, test x$enable_unittests != xno)
 AM_CONDITIONAL(COVERAGE_ENABLED, test x$enable_coverage = xyes)
@@ -56,6 +58,11 @@ if test "$enable_static_tor" = "yes"; then
   CFLAGS="$CFLAGS -static"
 fi
 
+if test "$enable_system_torrc" = "no"; then
+  AC_DEFINE(DISABLE_SYSTEM_TORRC, 1,
+            [Defined if we're not going to look for a torrc in SYSCONF])
+fi
+
 if test x$enable_buf_freelists = xyes; then
   AC_DEFINE(ENABLE_BUF_FREELISTS, 1,
             [Defined if we try to use freelists for buffer RAM chunks])

src/or/config.c

@@ -4013,7 +4013,10 @@ get_windows_conf_root(void)
 static const char *
 get_default_conf_file(int defaults_file)
 {
-#ifdef _WIN32
+#ifdef DISABLE_SYSTEM_TORRC
+  (void) defaults_file;
+  return NULL;
+#elif defined(_WIN32)
   if (defaults_file) {
     static char defaults_path[MAX_PATH+1];
     tor_snprintf(defaults_path, MAX_PATH, "%s\\torrc-defaults",