THRIFT-2925 CMake build does not work with OpenSSL nor anything installed in non-system location

This diff fixes them by correcting invalid CMake variable names:
* OPENSSL_... rather than OpenSSL_...
* LIBEVENT_... rather than Libevent_...
* Boost_INCLUDE_DIRS rather than Boost_INCLUDE_DIR
* LIBEVENT_INCLUDE_DIRS rather than LIBEVENT_INCLUDE_DIR
* ZLIB_INCLUDE_DIRS rather than ZLIB_INCLUDE_DIR

Note:
* OPENSSL_INCLUDE_DIR is correct (rather than ..._DIRS)
* Boost_INCLUDE_DIR exists and actually works for most cases but
  Boost_INCLUDE_DIRS is the one desinged to be included

Also, library headers are now included as SYSTEM headers.
diff --git a/cmake/DefineOptions.cmake b/cmake/DefineOptions.cmake
index d7144ea..391247e 100644
--- a/cmake/DefineOptions.cmake
+++ b/cmake/DefineOptions.cmake
@@ -59,7 +59,7 @@
 endif()
 find_package(OpenSSL QUIET)
 CMAKE_DEPENDENT_OPTION(WITH_OPENSSL "Build with OpenSSL support" ON
-                       "OpenSSL_FOUND" OFF)
+                       "OPENSSL_FOUND" OFF)
 option(WITH_BOOSTTHREADS "Build with Boost thread support" OFF)
 option(WITH_STDTHREADS "Build with C++ std::thread support" OFF)
 
diff --git a/lib/cpp/CMakeLists.txt b/lib/cpp/CMakeLists.txt
index 7838726..4c73986 100755
--- a/lib/cpp/CMakeLists.txt
+++ b/lib/cpp/CMakeLists.txt
@@ -26,7 +26,7 @@
 else()
   find_package(Boost 1.53.0 REQUIRED)
 endif()
-include_directories("${Boost_INCLUDE_DIR}")
+include_directories(SYSTEM "${Boost_INCLUDE_DIRS}")
 
 include_directories(src)
 
@@ -98,7 +98,7 @@
        src/thrift/transport/TSSLSocket.cpp
        src/thrift/transport/TSSLServerSocket.cpp
     )
-    include_directories("${OPENSSL_INCLUDE_DIR}")
+    include_directories(SYSTEM "${OPENSSL_INCLUDE_DIR}")
     list(APPEND SYSLIBS "${OPENSSL_LIBRARIES}")
 endif()
 
@@ -158,15 +158,15 @@
 
 if(WITH_LIBEVENT)
     find_package(Libevent REQUIRED)  # Libevent comes with CMake support form upstream
-    include_directories(${Libevent_INCLUDE_DIR})
+    include_directories(SYSTEM ${LIBEVENT_INCLUDE_DIRS})
 
     ADD_LIBRARY_THRIFT(thriftnb ${thriftcppnb_SOURCES})
-    TARGET_LINK_LIBRARIES_THRIFT(thriftnb ${SYSLIBS} ${Libevent_LIBRARIES})
+    TARGET_LINK_LIBRARIES_THRIFT(thriftnb ${SYSLIBS} ${LIBEVENT_LIBRARIES})
 endif()
 
 if(WITH_ZLIB)
     find_package(ZLIB REQUIRED)
-    include_directories(${ZLIB_INCLUDE_DIR})
+    include_directories(SYSTEM ${ZLIB_INCLUDE_DIRS})
 
     ADD_LIBRARY_THRIFT(thriftz ${thriftcppz_SOURCES})
     TARGET_LINK_LIBRARIES_THRIFT(thriftz ${SYSLIBS} ${ZLIB_LIBRARIES})
diff --git a/lib/cpp/test/CMakeLists.txt b/lib/cpp/test/CMakeLists.txt
index b524dd7..d218da9 100644
--- a/lib/cpp/test/CMakeLists.txt
+++ b/lib/cpp/test/CMakeLists.txt
@@ -21,7 +21,7 @@
 # Find required packages
 set(Boost_USE_STATIC_LIBS ON) # Force the use of static boost test framework
 find_package(Boost 1.53.0 REQUIRED COMPONENTS unit_test_framework)
-include_directories("${Boost_INCLUDE_DIR}")
+include_directories(SYSTEM "${Boost_INCLUDE_DIRS}")
 
 #Make sure gen-cpp files can be included
 include_directories("${CMAKE_CURRENT_BINARY_DIR}")