Revert "Remove pkcs11-helper option"

This reverts commit d832f187f756079552601867348d924582bf65de.

Conflicts:
* CMakeLists.txt:
  * USE_PKCS11_HELPER_LIBRARY: there has been a change immediately before
    where it was removed. Just re-add what was removed.
* tests/CMakeLists.txt:
  * USE_PKCS11_HELPER_LIBRARY: there has been a change immediately before
    where it was removed. Just re-add what was removed.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index ecc33ee..5cc7a0a 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -9,6 +9,10 @@
     set(MBEDTLS_DIR ${CMAKE_SOURCE_DIR})
 endif()
 
+if(USE_PKCS11_HELPER_LIBRARY)
+    set(libs ${libs} pkcs11-helper)
+endif(USE_PKCS11_HELPER_LIBRARY)
+
 if(ENABLE_ZLIB_SUPPORT)
     set(libs ${libs} ${ZLIB_LIBRARIES})
 endif(ENABLE_ZLIB_SUPPORT)
diff --git a/tests/Makefile b/tests/Makefile
index cca7c1c..ae9ba85 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -1,5 +1,6 @@
 
 # To compile on SunOS: add "-lsocket -lnsl" to LDFLAGS
+# To compile with PKCS11: add "-lpkcs11-helper" to LDFLAGS
 
 CFLAGS	?= -O2
 WARNING_CFLAGS ?= -Wall -Wextra