build: only attempt to install MSVC system libraries on Windows (#6)

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

@@ -115,8 +115,10 @@ if (MSVC)
     APPEND PROPERTY LINK_FLAGS /INCREMENTAL:NO)
 endif(MSVC)
 
-set(CMAKE_INSTALL_SYSTEM_RUNTIME_LIBS_NO_WARNINGS ON)
-include (InstallRequiredSystemLibraries)
+if(NOT MSVC OR CMAKE_HOST_SYSTEM_NAME STREQUAL "Windows")
+  set(CMAKE_INSTALL_SYSTEM_RUNTIME_LIBS_NO_WARNINGS ON)
+  include(InstallRequiredSystemLibraries)
+endif()
 install(TARGETS ${PROGRAM} ${LIBRARY} ${STATICLIBRARY}
   RUNTIME DESTINATION bin
   LIBRARY DESTINATION lib