Fix building without duktape
Fix building without duktape

Otherwise configuring fails with

CMake Error at libproxy/cmake/modules.cmk:26 (math):
  math cannot parse the expression: "0+0+0+": syntax error, unexpected end of
  file (6).
Call Stack (most recent call first):
  libproxy/CMakeLists.txt:14 (include)

libproxy/libproxy,libproxy/libproxy,libproxy/libproxy,libproxy/libproxy,libproxy/libproxy,libproxy/libproxy,libproxy/libproxy