meson: Throw error if built with LTO enabled

Right now, link-time optimisation doesn't work and produces symbols with
null references with our linker script.

For example without LTO:

  $ nm -D result/lib/libip2unix.so | grep socket
  000000000000fed0 T socket

With LTO enabled however, we get the following:

  $ nm -D result/lib/libip2unix.so | grep socket
  0000000000000000 A socket

I haven't managed to find out why *exactly* this happens, but since we
don't have a fix yet, let's at least make sure users get an error
message early on instead of a crash.

Signed-off-by: aszlig <aszlig@nix.build>
Issue: #33

meson.build

@@ -20,6 +20,11 @@ if std_fs_check.returncode() != 0
         ' please update your compiler toolchain.')
 endif
 
+if get_option('b_lto')
+  error('Link-time optimisation is not supported,' +
+        ' please build with b_lto set to false (-Db_lto=false)')
+endif
+
 warning_flags = [
   '-Wcast-qual',
   '-Wcovered-switch-default',