Set OPENSSL_MSVC_STATIC_RT to value of MSVC_CRT_STATIC and update manual windows building instructions

ValveSoftware/GameNetworkingSockets#232

BUILDING_WINDOWS_MANUAL.md

@@ -48,6 +48,14 @@ easier. Be sure to pick the installers **without** the "Light"suffix. In this in
 For CMake to find the libraries, you may need to set the environment variable
 `OPENSSL_ROOT_DIR`.
 
+If you are linking statically with OpenSSL you will need to set `OPENSSL_USE_STATIC_LIBS` to `ON`.
+`OPENSSL_MSVC_STATIC_RT` is automatically matched to the value of `MSVC_CRT_STATIC`.
+
+Both need to be set before any calls to `find_package(OpenSSL REQUIRED)` because CMake caches the paths to libraries.
+When building GameNetworkingSockets by itself it is sufficient to set these variables on the command line or in the GUI before first configuration.
+
+See the documentation for [FindOpenSSL](https://cmake.org/cmake/help/latest/module/FindOpenSSL.html) for more information about CMake variables involing OpenSSL.
+
 #### Protobuf
 
 Instructions for getting a working installation of google protobuf on Windows can

CMakeLists.txt

@@ -97,6 +97,12 @@ if (WIN32)
 endif()
 
 if (USE_CRYPTO STREQUAL "OpenSSL")
+	# Match the OpenSSL runtime to our setting.
+	# Note that once found the library paths are cached and will not change if the option is changed.
+	if (MSVC)
+		set(OPENSSL_MSVC_STATIC_RT ${MSVC_CRT_STATIC})
+	endif()
+
 	find_package(OpenSSL REQUIRED)
 	message( STATUS "OPENSSL_INCLUDE_DIR = ${OPENSSL_INCLUDE_DIR}" )