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

CMakeLists.txt

@@ -68,7 +68,9 @@ set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 string(REPLACE "-O3" "-O2" CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE}")
 
 # Enable -Wconversion.
-set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wconversion")
+if(NOT MSVC)
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wconversion")
+endif()
 
 # gcc 4.0 and better turn on _FORTIFY_SOURCE=2 automatically.  This currently
 # does not work with Neovim due to some uses of dynamically-sized structures.
@@ -110,8 +112,14 @@ if(NOT HAS_ACCEPTABLE_FORTIFY)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${_FORTIFY_SOURCE_PREFIX}-U_FORTIFY_SOURCE ${_FORTIFY_SOURCE_PREFIX}-D_FORTIFY_SOURCE=1")
 endif()
 
-add_definitions(-Wall -Wextra -pedantic -Wno-unused-parameter
+if(MSVC)
+  # XXX: /W4 gives too many warnings. #3241
+  add_definitions(/W3 -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_DEPRECATE)
+else()
+  add_definitions(-Wall -Wextra -pedantic -Wno-unused-parameter
     -Wstrict-prototypes -std=gnu99)
+endif()
+
 if(MINGW)
   # Use POSIX compatible stdio in Mingw
   add_definitions(-D__USE_MINGW_ANSI_STDIO)

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)