Fixes some errors on Linux system

Hi I've encountered some error while doing:

$ cmake .
CMake Error at /usr/share/cmake-3.9/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
  Could NOT find OpenCL (missing: OpenCL_LIBRARY) (found version "2.0")
Call Stack (most recent call first):
  /usr/share/cmake-3.9/Modules/FindPackageHandleStandardArgs.cmake:377 (_FPHSA_FAILURE_MESSAGE)
  cmake/FindOpenCL.cmake:143 (find_package_handle_standard_args)
  CMakeLists.txt:7 (find_package)

-- Configuring incomplete, errors occurred!
See also "/home/debian/Build/MINING/orig/gpuPlotGenerator/CMakeFiles/CMakeOutput.log".
See also "/home/debian/Build/MINING/orig/gpuPlotGenerator/CMakeFiles/CMakeError.log".

Apparently if(${CMAKE_SYSTEM_NAME} MATCHES "Windows") evaluates to "True" when cmake starts finding the OpenCL_LIBRARIES.`

=======

And the other commit fixes the error when gpuPlotGenerator generate direct fails to write on the preallocated plot file.

./gpuPlotGenerator generate direct 32423423423_0_4096_4096
-------------------------
GPU plot generator v4.1.3
-------------------------
Author:   Cryo
Bitcoin:  138gMBhCrNkbaiTCmUhP9HLU9xwn5QKZgD
Burst:    BURST-YA29-QCEW-QXC3-BKXDL
----
Loading platforms...
Loading devices...
Loading devices configurations...
Initializing generation devices...
    [0] Device: Ellesmere (OpenCL 1.2 AMD-APP (2348.3))
    [0] Device memory: 256MB
    [0] CPU memory: 256MB
Initializing generation contexts...

[ERROR] Unable to open plots file

when checking on the file:

ls -l 32423423423_0_4096_4096 
---------- 1 user user 1073741824 Aug  3 02:37 32423423423_0_4096_4096

bhamon / gpuPlotGenerator

Fixes some errors on Linux system #24