Fix TRANTOR_USE_TLS which should not be using option but set the cache directly.

Author: Lectem

CMakeLists.txt

@@ -5,9 +5,9 @@ option(BUILD_DOC "Build Doxygen documentation" OFF)
 option(BUILD_C-ARES "Build C-ARES" ON)
 option(BUILD_TESTING "Build tests" OFF)
 option(BUILD_SHARED_LIBS "Build trantor as a shared lib" OFF)
-option(TRANTOR_USE_TLS
-       "TLS provider for trantor. Valid options are 'openssl', 'botan' or '' (let the build scripr decide)" ""
-)
+set(TRANTOR_USE_TLS "" CACHE STRING
+    "TLS provider for trantor. Valid options are 'openssl', 'botan', 'none' or '' (auto-detect)")
+set_property(CACHE TRANTOR_USE_TLS PROPERTY STRINGS "" openssl botan none)
 option(USE_SPDLOG "Allow using the spdlog logging library" OFF)
 
 list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake_modules/)
@@ -158,10 +158,6 @@ else(WIN32)
   set(TRANTOR_SOURCES ${TRANTOR_SOURCES} trantor/net/inner/FileBufferNodeUnix.cc)
 endif(WIN32)
 
-# Somehow the default value of TRANTOR_USE_TLS is OFF
-if(TRANTOR_USE_TLS STREQUAL OFF)
-  set(TRANTOR_USE_TLS "")
-endif()
 set(VALID_TLS_PROVIDERS "openssl" "botan" "none")
 list(
   FIND