MSVC: use MSVC instead of GCC-like compiler flags.

- We can build with /W4, but it gives thousands of warnings.
- _CRT_NONSTDC_NO_DEPRECATE disables warnings about POSIX functions.
  https://msdn.microsoft.com/en-us/library/ms235384.aspx
- _CRT_SECURE_NO_WARNINGS
  https://msdn.microsoft.com/en-us/library/8ef0s5kh.aspx

src/nvim/CMakeLists.txt

@@ -101,8 +101,10 @@ foreach(sfile ${CONV_SOURCES})
   endif()
 endforeach()
 
-set_source_files_properties(
-  ${CONV_SOURCES} PROPERTIES COMPILE_FLAGS "${COMPILE_FLAGS} -Wno-conversion")
+if(NOT MSVC)
+  set_source_files_properties(
+    ${CONV_SOURCES} PROPERTIES COMPILE_FLAGS "${COMPILE_FLAGS} -Wno-conversion")
+endif()
 
 get_directory_property(gen_cdefs COMPILE_DEFINITIONS)
 foreach(gen_cdef ${gen_cdefs} DO_NOT_DEFINE_EMPTY_ATTRIBUTES)