"" cmake prefix - required on POSIX

ipaacalib/cpp/build.xml

@@ -26,7 +26,9 @@
          <generator name="Visual Studio 10" platform="windows" buildargs="ALL_BUILD.vcxproj">
              <variable name="CMAKE_INSTALL_PREFIX" type="PATH" value="../${zip.tmp.dir}" />
          </generator>
-         <generator name="Unix Makefiles" />
+         <generator name="Unix Makefiles">
+             <variable name="CMAKE_INSTALL_PREFIX" type="PATH" value="" />
+         </generator>
          <variable name="SPREAD_ROOT" type="FILEPATH" value="${env.SPREAD_ROOT}" />
          <variable name="PROTOBUF_ROOT" type="FILEPATH" value="${env.PROTOBUF_ROOT}" />
       </cmake>