CMake: Detect we are using static libs for Boost

Previously we relied on the user to supply Boost_USE_STATIC_LIBS in
order to link Boost dependencies.

CMakeLists.txt

@@ -123,7 +123,8 @@ list(APPEND LIBRARIES ${Boost_LIBRARIES})
 link_directories(${Boost_LIBRARY_DIRS})
 include_directories(SYSTEM ${Boost_INCLUDE_DIR})
 
-if(Boost_USE_STATIC_LIBS)
+has_static_libs(Boost Boost_LIBRARIES)
+if(Boost_HAS_STATIC_LIBS)
 	
 	use_static_libs(ZLIB)
 	find_package(ZLIB REQUIRED)

cmake/UseStaticLibs.cmake

@@ -44,3 +44,18 @@ macro(use_static_libs_restore)
 		unset(_UseStaticLibs_ORIG_CMAKE_FIND_LIBRARY_SUFFIXES)
 	endif()
 endmacro()
+
+macro(has_static_libs PREFIX LIBS)
+	if(WIN32)
+		# On Windows we can't really tell import libraries from proper static libraries.
+		set(${PREFIX}_HAS_STATIC_LIBS ${${PREFIX}_USE_STATIC_LIBS})
+	else()
+		set(${PREFIX}_HAS_STATIC_LIBS 0)
+		foreach(lib IN LISTS ${LIBS})
+			if(lib MATCHES "\\.a$")
+				set(${PREFIX}_HAS_STATIC_LIBS 1)
+				break()
+			endif()
+		endforeach()
+	endif()
+endmacro()