feature: option to build tcvm without skia

This feature ables to build TCVM without Skia. By default, this option is ON.
Without Skia, is possible to generate a very little final libtcvm
(~3MB on Linux).
It's also now possible to test the VM on OpenBSD.
Usage - Disable Skia: cmake <source_dir> -DUSE_SKIA=OFF

TotalCrossVM/CMakeLists.txt

@@ -34,7 +34,7 @@ ENDIF (NOT (DEFINED ANDROID_ABI OR MSVC OR CMAKE_GENERATOR STREQUAL Xcode))
 if(NOT MSVC AND USE_SKIA)
   find_package(Skia REQUIRED)
   include_directories(${SKIA_INCLUDE_DIRS})
-  add_compile_definitions(USE_SKIA)
+  add_definitions(-DUSE_SKIA)
 endif(NOT MSVC AND USE_SKIA)
 
 set(TC_SRCDIR "${CMAKE_CURRENT_SOURCE_DIR}/src")