Build break fix suggestions for SDK 1.5.0 when compiling it as DLL (MultiThreadedDLL) in VS2015

maybe its useful for someone...

due to i needed to include the kernel lib within a dll, some adoptions to the project file and OVR_allocator.cpp had to be done.

- libOVRKernel.vcxproj: 
  Change all <RuntimeLibrary>MultiThreaded*</RuntimeLibrary> into <RuntimeLibrary>MultiThreaded*Dll</RuntimeLibrary>
  this Adoption also was needed in SDK 1.4.0 (maybe also before, but within 0.6.01 it was not, which was the previous version i had integraded into our SW)

- Adoptions / fixes in OVR_allocator.cpp (new adoptions since 1.5.0)
  * imho according to line 163, line 176 should be "_MSC_VER >= 1900" instead of "_MSC_VER < 1900"
  * line 267-271 missing "_ACRTIMP" define in front of return value of the external declarations (e.g. extern "C" _ACRTIMP void  __cdecl _free_base(void* p))
  * missing ending ";" in line 3316
  * changed value of OVR_DEBUG_CRT_PRESENT definition in line 64 from 1 to 0
  * changed value of OVR_ALLOCATOR_TRACKING_ENABLED Definition in line 116 from 1 to 0

These changes allowed me to build and link the SDK against a DLL of our own