Allow the user to build oqs-provider as a static library.

This commit removes the `SHARED` argument of the `add_library`.
By doing so, we let the user choose the build type of library.

By default, CMake will build a static library. Thus, [`BUILD_SHARED_LIBS`]
must be used to switch to a shared library.

`oqs-provider` as a static library allows us to use the provider without
having to store its shared library somewhere. In addition, it happens that
some operating systems prohibit the use of `dlopen`/`dlsym`.

To load `oqs-provider` when it is embedded into a library of a binary, one
can use the [`OSSL_PROVIDER_add_builtin`] API from OpenSSL 3.

[`BUILD_SHARED_LIBS`]: https://cmake.org/cmake/help/latest/variable/BUILD_SHARED_LIBS.html
[`OSSL_PROVIDER_add_builtin`]: https://www.openssl.org/docs/man3.1/man3/OSSL_PROVIDER_add_builtin.html

oqsprov/CMakeLists.txt

@@ -31,7 +31,7 @@ set(PROVIDER_SOURCE_FILES
 set(PROVIDER_HEADER_FILES
   oqs_prov.h oqs_endecoder_local.h
 )
-add_library(oqsprovider SHARED ${PROVIDER_SOURCE_FILES})
+add_library(oqsprovider ${PROVIDER_SOURCE_FILES})
 if (USE_ENCODING_LIB)
   add_dependencies(oqsprovider encoder)
 endif()

scripts/fullbuild.sh

@@ -107,9 +107,9 @@ if [ ! -f "_build/lib/oqsprovider.$SHLIBEXT" ]; then
    BUILD_TYPE=""
    # for omitting public key in private keys add -DNOPUBKEY_IN_PRIVKEY=ON
    if [ -z "$OPENSSL_INSTALL" ]; then
-       cmake -DOPENSSL_ROOT_DIR=$(pwd)/.local $BUILD_TYPE -S . -B _build && cmake --build _build
+       cmake -DOPENSSL_ROOT_DIR=$(pwd)/.local $BUILD_TYPE -S . -B _build -DBUILD_SHARED_LIBS=ON && cmake --build _build
    else
-       cmake -DOPENSSL_ROOT_DIR=$OPENSSL_INSTALL $BUILD_TYPE -S . -B _build && cmake --build _build
+       cmake -DOPENSSL_ROOT_DIR=$OPENSSL_INSTALL $BUILD_TYPE -S . -B _build -DBUILD_SHARED_LIBS=ON && cmake --build _build
    fi
    if [ $? -ne 0 ]; then
      echo "provider build failed. Exiting."