fix use of CMAKE_SYSTEM_PROCESSOR

Hi,

When compiling from linux CMAKE_SYSTEM_PROCESSOR is emtpy and cmake fails: 
Make Error at CMakeLists.txt:378 (if):
  if given arguments:

    "MATCHES" "i386" "OR" "MATCHES" "i486" "OR" "MATCHES" "i586" "OR" "MATCHES" "i686" "OR" "MATCHES" "x86" "OR" "MATCHES" "x86_64" "OR" "MATCHES" "amd64"

  Unknown arguments specified

See:
http://cmake.org/cmake/help/v2.8.12/cmake.html#variable:CMAKE_SYSTEM_PROCESSOR

This patch uses CMAKE_HOST_SYSTEM_PROCESSOR if CMAKE_SYSTEM_PROCESSOR is empty.
Or we could use CMAKE_HOST_SYSTEM_PROCESSOR all the way, did you tried compiling from cygwin when writing this code ?

Anyway I feel the test is useless as mingw is not available on anything else than x86 archs afaik.

Looking forward to discuss this.

xan.