Disable MSVC warning 4251 in header rather than in CMakeLists

CMakeLists.txt

@@ -36,7 +36,7 @@ set(CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake/Modules)
 
 if(WIN32)
 	add_definitions(-DWIN32_LEAN_AND_MEAN)
-	if (MSVC)
+	if(MSVC)
 		add_definitions(-DNOMINMAX)
 		add_definitions(-D_CRT_SECURE_NO_WARNINGS)
 		add_definitions(-D_SILENCE_CXX17_CODECVT_HEADER_DEPRECATION_WARNING)
@@ -247,10 +247,7 @@ add_library(LibDataChannel::LibDataChannelStatic ALIAS datachannel-static)
 install(TARGETS datachannel LIBRARY DESTINATION lib)
 install(FILES ${LIBDATACHANNEL_HEADERS} DESTINATION include/rtc)
 
-if(MSVC)
-	target_compile_options(datachannel PRIVATE /wd4251)
-	target_compile_options(datachannel-static PRIVATE /wd4251)
-else()
+if(NOT MSVC)
 	target_compile_options(datachannel PRIVATE -Wall -Wextra)
 	target_compile_options(datachannel-static PRIVATE -Wall -Wextra)
 endif()

include/rtc/include.hpp

@@ -32,6 +32,9 @@
 #ifndef _WIN32_WINNT
 #define _WIN32_WINNT 0x0602 // Windows 8
 #endif
+#ifdef _MSC_VER
+#pragma warning(disable:4251) // disable "X needs to have dll-interface..."
+#endif
 #else
 #define RTC_CPP_EXPORT
 #endif