cmake: Add MSVC flags for Wall, Wno-sign-compare and Werror

This will bring the Windows AppVeyor builds to the same as the other
CI infrastructure, and will stop and error on any warnings.

Signed-off-by: Robin Getz <robin.getz@analog.com>

CMakeLists.txt

@@ -54,9 +54,16 @@ if (WITH_TESTS)
 endif()
 
 if (MSVC)
-	# Avoid annoying warnings from Visual Studio
-	add_definitions(-D_CRT_SECURE_NO_WARNINGS=1)
-
+	add_compile_options(/W4 /wd4018 /wd4200 /wd4127 /wd4100)
+	# C4018: signed/unsigned mismatch ; same as -Wno-sign-compare
+	# C4200: nonstandard extension used : zero-sized array in struct (usb.h)
+	# C4127: conditional expression is constant (IIO_ERROR and IIO_DEBUG macros)
+	# C4100: unreferenced parameter; same as -Wno-unused-parameter
+
+	if(DEFINED ENV{CI} AND DEFINED ENV{APPVEYOR})
+		message(STATUS "Running in an AppVeyor environment, setting -Werror")
+		add_compile_options(/WX)
+	endif()
 	set(CMAKE_FIND_LIBRARY_PREFIXES "lib" "")
 	set(CMAKE_FIND_LIBRARY_SUFFIXES ".dll.a" ".a" ".lib")
 endif()