Adjust README to reflect change regarding b_lundef

With Meson >=0.48.0 it is no longer necessary to pass the flag
"-D b_lundef=false" on FreeBSD.

README.md

@@ -74,8 +74,8 @@ Run these commands:
     meson build
     ninja -C build
 
-On FreeBSD, you need to pass an extra flag to prevent a linking error:
-`meson build -D b_lundef=false`.
+If you use Meson older than 0.48.0 on FreeBSD, you need to pass an extra flag
+to prevent a linking error: `meson build -D b_lundef=false`.
 
 Install like so: