feat(build): add option to use system OpenSSL

* Introduced `GPGFRONTEND_USE_SYSTEM_OPENSSL` option to allow
  using the system-installed OpenSSL instead of building it.
* Updated CMake configuration to handle OpenSSL linking based
  on the new option.
* Ensured static libraries are used for OpenSSL to support
  application self-checking and avoid potential issues with
  shared library loading.

Author: saturneric

CMakeLists.txt

@@ -70,6 +70,7 @@ option(GPGFRONTEND_XCODE_ENABLE_SANDBOX "Enable SandBox For Xcode Build" OFF)
 
 # check options
 option(GPGFRONTEND_LINK_GPGME_INTO_CORE "Statically link GpgME into the core library" OFF)
+option(GPGFRONTEND_USE_SYSTEM_OPENSSL "Use system OpenSSL instead of building it" OFF)
 option(GPGFRONTEND_RUST_SUPPORT "Enable Rust support for GpgFrontend" OFF)
 
 if(GPGFRONTEND_BUILD_APP_IMAGE)
@@ -107,6 +108,12 @@ else()
   set(BUILD_AND_STATIC_LINK_GPGME 0)
 endif()
 
+if(GPGFRONTEND_USE_SYSTEM_OPENSSL)
+  set(USE_SYSTEM_OPENSSL 1)
+else()
+  set(USE_SYSTEM_OPENSSL 0)
+endif()
+
 if (GPGFRONTEND_RUST_SUPPORT)
   set(BUILD_RUST_SUPPORT 1)
 else()
@@ -247,6 +254,12 @@ if(MINGW)
 
   set(OS_PLATFORM 0)
 
+  # for mingw, we will always use the system openssl, since it's easier to get
+  # it working on windows and the openssl project provides official pre-built
+  # binaries for windows. And in the msys2 environment, the openssl is also
+  # available as a package, and it's often the latest version.
+  set(USE_SYSTEM_OPENSSL 1)
+
   include_directories(
     ${CMAKE_CURRENT_SOURCE_DIR}/src
     ${CMAKE_CURRENT_SOURCE_DIR}/third_party

third_party/CMakeLists.txt

@@ -201,21 +201,19 @@ if(BUILD_AND_STATIC_LINK_GPGME AND XCODE_BUILD)
   target_link_libraries(GnuPGDeps INTERFACE ${GPGME_STATIC_LIB} ${ASSUAN_STATIC_LIB} ${GPG_ERROR_STATIC_LIB})
 endif()
 
-# Introduce OpenSSL
-if(MINGW)
-  # since OpenSSL on msys2 is often the latest version and is well maintained ,
-  # and it's better to use it to avoid build issues.
+# Introduce OpenSSL targets
+if(USE_SYSTEM_OPENSSL)
   set(SYSTEM_OPENSSL ON)
   set(BUILD_OPENSSL OFF)
-  # static linking on Windows for better security and portability, and it's
-  # generally recommended for OpenSSL on Windows.
-  set(OPENSSL_USE_STATIC_LIBS ON)
 else()
   set(SYSTEM_OPENSSL OFF)
   set(BUILD_OPENSSL ON)
 endif()
 
 set(OPENSSL_BUILD_VERSION "3.6.1")
+# We will always use static libraries for openssl for the support of application
+# self-checking. This may avoid the potential issues of corrupted shared library
+# loading.
 set(OPENSSL_USE_STATIC_LIBS ON)
 add_subdirectory(openssl-cmake)