Revert "Remove check for boost.thread from buildsystems since it's not used anymore"

This reverts commit 4c74977227.
Explicit linking against boost_thread is required when using boost as
static libraries.

CMakeLists.txt

@@ -75,7 +75,7 @@ else()
 	set(CRYPTO_LIBRARY "-framework Security")
 endif()
 
-find_package(Boost 1.56 REQUIRED COMPONENTS iostreams program_options regex system random)
+find_package(Boost 1.56 REQUIRED COMPONENTS iostreams program_options regex system thread random)
 
 # no, gettext executables are not required when NLS is deactivated
 find_package(Gettext)

SConstruct

@@ -367,6 +367,7 @@ if env["prereqs"]:
         conf.CheckBoost("random", require_version = boost_version) & \
         conf.CheckBoost("smart_ptr", header_only = True) & \
 	CheckAsio(conf) & \
+	conf.CheckBoost("thread") & \
         conf.CheckBoost("locale") & \
         conf.CheckBoost("filesystem") \
             and Info("Base prerequisites are met")) \

src/CMakeLists.txt

@@ -76,6 +76,7 @@ set(game-external-libs
 	${CRYPTO_LIBRARY}
 	${Boost_SYSTEM_LIBRARIES}
 	${Boost_RANDOM_LIBRARY}
+	${Boost_THREAD_LIBRARY}
 	${sdl_image-lib}
 	${sdl_mixer-lib}
 	${sdl_ttf-lib}