build: only attempt to install MSVC system libraries on Windows

Newer versions of CMake attempt to query the system for information about the VS
2017 installation.  Unfortunately, this query fails on non-Windows systems when
cross-compiling:

	cmake_host_system_information does not recognize <key> VS_15_DIR

CMake will not find these system libraries on non-Windows hosts anyways, and we
were silencing the warnings, so simply omit the installation when
cross-compiling to Windows.

Author: compnerd

src/CMakeLists.txt

@@ -128,11 +128,13 @@ if (MSVC)
     APPEND PROPERTY LINK_FLAGS /INCREMENTAL:NO)
 endif(MSVC)
 
-set(CMAKE_INSTALL_SYSTEM_RUNTIME_LIBS_NO_WARNINGS ON)
+if(NOT MSVC OR CMAKE_HOST_SYSTEM_NAME STREQUAL Windows)
+  set(CMAKE_INSTALL_SYSTEM_RUNTIME_LIBS_NO_WARNINGS ON)
+  include(InstallRequiredSystemLibraries)
+endif()
 
 set(libdir lib${LIB_SUFFIX})
 
-include (InstallRequiredSystemLibraries)
 install(TARGETS ${PROGRAM} ${CMARK_INSTALL}
   EXPORT cmark
   RUNTIME DESTINATION bin