Just fix opus-uninstalled.pc instead of tweaking the build dir

Doing it this way means it's only the weird corner case that actually
pays for being weird, and we don't litter the build dir with an extra
link that normal builds really don't need at all.

It also avoids the problem of platforms where LN_S isn't well defined.

configure.ac

@@ -292,13 +292,6 @@ AC_SUBST([PC_BUILD])
 AC_SUBST([PC_LIBM])
 
 
-AC_CONFIG_COMMANDS([builddir-include],
-                   [ if test ! -e include; then
-                        $LN_S $ac_top_srcdir/include include
-                     fi
-                   ],[ LN_S="$LN_S" ]
-                  )
-
 AC_CONFIG_FILES([Makefile opus.pc opus-uninstalled.pc
                  doc/Makefile doc/Doxyfile])
 

opus-uninstalled.pc.in

@@ -9,4 +9,4 @@ Version: @VERSION@
 Requires:
 Conflicts:
 Libs: ${libdir}/libopus.a @PC_LIBM@
-Cflags: -I${includedir}/include
+Cflags: -I${pcfiledir}/@top_srcdir@/include