Revert "pthread device: increased minimum buffer allocation size to 10MB"

This reverts commit 3ebd0d9ea1a4ec2fc8191051ce6c84e7cdcac2fa.
Add a minimum memory region size to avoid ending up with
a bunch of memory regions in case of applications with
very small buffers.
Added Visual Studio compatible alignment macros.
Removed ltdl.h when compiling with Visual Studio.
Added Visual Studio implementation for __restrict__
Added Visual Studion implementations for libltdl functions.
Undeclared a define made in Windows.h which conflicts with a goto label.
Fixed preprocessor or/and/not operators to be vc++ compatible.
Added VC++ implementation for lt_dlinit and removed hidden visibility pragma from GET_PROTOTYPES macro.
Removed header from build list to prevent triggering #error in Visual Studio.
Visual Studio does not have include_next.
Added include to get cl_device_id typedef.
Fixed places where unistd.h has been included to include vccompat.hpp on Visual Studio.
Fixed last missing Visual Studio header.
Added snprintf compatibility define.
Fixed allocating dynamic size array to use malloc.
Added strtok_r implementation for Visual Studio.
Fixed casting malloc return type for pthread.c and added alloca compatibility code for Visual Studio.
Make building of pocl-devices library to pass on Visual Studio.
Added many explicit void* casting to actual pointer for Visual Studio and implemented couple of missing functions.