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/CMakeLists.txt b/CMakeLists.txt
index 4d40eea..9c6fb5c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -8,6 +8,7 @@
 # Set the project root directory.
 set(MBEDTLS_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 
+option(USE_PKCS11_HELPER_LIBRARY "Build mbed TLS with the pkcs11-helper library." OFF)
 option(ENABLE_ZLIB_SUPPORT "Build mbed TLS with zlib library." OFF)
 
 option(ENABLE_PROGRAMS "Build mbed TLS programs." ON)
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index a602a6b..0053f09 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -148,6 +148,10 @@
     set(libs ${libs} network)
 endif(HAIKU)
 
+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/programs/Makefile b/programs/Makefile
index 65a31b1..feec288 100644
--- a/programs/Makefile
+++ b/programs/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
diff --git a/programs/test/CMakeLists.txt b/programs/test/CMakeLists.txt
index 0d2b946..64ed379 100644
--- a/programs/test/CMakeLists.txt
+++ b/programs/test/CMakeLists.txt
@@ -2,6 +2,10 @@
     mbedtls
 )
 
+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/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