CMAKE: look for pthreads when importing wolfSSL if required

All required dependencies of a package must also be found in the
package configuration file. Consumers of wolfSSL can't know
if it was built with or without threads support. This change
adds find_package(Threads) lookup in the file used for
find_package(wolfssl) if wolfSSL was built with threads support.

cmake/Config.cmake.in

@@ -1,3 +1,9 @@
 @PACKAGE_INIT@
 
+include(CMakeFindDependencyMacro)
+if (@HAVE_PTHREAD@)
+  find_dependency(Threads)
+endif()
+
+
 include ( "${CMAKE_CURRENT_LIST_DIR}/wolfssl-targets.cmake" )