On Solaris, overwrite CMake's proposed thread library -lthread with -lpthread.

-lthread works fine in most cases, but at least with gcc 3.4.6 on x86, dlopen() crashes when libpthread is not used. 
Note : the workaround existed prior and did not work  since CMAKE_THREADS_LIBS_INIT was already
 in cache.  Now, use SET(.. CACHE FORCE)  to overwrite the cached value.

cmake/os/SunOS.cmake

@@ -30,7 +30,7 @@ SET(LIBM m)
 
 # CMake defined -lthread as thread flag. This crashes in dlopen 
 # when trying to load plugins workaround with -lpthread
-SET(CMAKE_THREADS_LIBS_INIT -lpthread CACHE INTERNAL "")
+SET(CMAKE_THREADS_LIBS_INIT -lpthread CACHE INTERNAL "" FORCE)
 
 # Solaris specific large page support
 CHECK_SYMBOL_EXISTS(MHA_MAPSIZE_VA sys/mman.h  HAVE_DECL_MHA_MAPSIZE_VA)