Mayaqua build: allow disabling OQS

SoftEtherVPN version 5.02.5186 enable post-quantum algorithms, but these
come at a large size increase (after strip, on x86_64, with default
options as of master):
- default options: 9.1M
- new -DOQS_ENABLE=OFF: 762K

Note it is also possible to disable all the algorithms individually by
passing the (243!) options to cmake -DOQS_ENABLE_KEM_BIKE=OFF
-DOQS_ENABLE_KEM_FRODOKEM=OFF -DOQS_ENABLE_KEM_NTRUPRIME=OFF ...,
in which case the binary goes back to a reasonable size of 830K

In the future, it might make sense to add a few settings picking
"sensible" algorithms, e.g. allow everything for a server build or only
allow the best algorithms for a lightweight client.

See: #2148

src/Mayaqua/CMakeLists.txt

@@ -18,9 +18,14 @@ set_target_properties(mayaqua
 
 find_package(OpenSSL REQUIRED)
 
-if(OPENSSL_VERSION VERSION_LESS "3") # Disable oqsprovider when OpenSSL version < 3
-  add_definitions(-DSKIP_OQS_PROVIDER)
+if(OPENSSL_VERSION VERSION_GREATER_EQUAL "3")
+  set(OQS_ENABLE ON CACHE BOOL "By setting this to OFF, Open Quantum Safe algorithms will not be built in")
 else()
+  # Disable oqsprovider when OpenSSL version < 3
+  set(OQS_ENABLE OFF)
+endif()
+
+if(OQS_ENABLE)
   set(OQS_BUILD_ONLY_LIB ON CACHE BOOL "Set liboqs to build only the library (no tests)")
   set(BUILD_TESTING OFF CACHE BOOL "By setting this to OFF, no tests or examples will be compiled.")
   set(OQS_PROVIDER_BUILD_STATIC ON CACHE BOOL "Build a static library instead of a shared library") # Build oqsprovider as a static library (defaults to shared)
@@ -32,6 +37,8 @@ else()
   target_include_directories(oqsprovider PUBLIC ${CMAKE_CURRENT_BINARY_DIR}/3rdparty/liboqs/include)
   set_property(TARGET oqsprovider PROPERTY POSITION_INDEPENDENT_CODE ON)
   target_link_libraries(mayaqua PRIVATE oqsprovider)
+else()
+  add_definitions(-DSKIP_OQS_PROVIDER)
 endif()
 
 include(CheckSymbolExists)