Fix build on Linux (Ubuntu)

In order to statically link the libssh2 library from the `.deps` folder using
the ExternalProjects module for Cmake, the static library needs to be compiled
with the `-fPIC` flag. In Cmake, this is the `CMAKE_POSITION_INDEPENDENT_CODE`
property and it can be set using the `-D` flag in the `CMAKE_ARGS` property for
the external project. Note, the `CMAKE_POSITION_INDEPENDENT_CODE` property is
`ON` by default for a _shared_ library, but it is `OFF` by default for a
_static_ library. Thus, it needs to be explicitly set to `ON` for the static
linking on Linux system.

CMakeLists.txt

@@ -83,7 +83,7 @@ if(BUILD_DEPS)
       UPDATE_COMMAND ""
       CMAKE_GENERATOR ${LIBSSH2_CMAKE_GENERATOR}
       CMAKE_GENERATOR_PLATFORM ${LIBSSH2_CMAKE_GENERATOR_PLATFORM}
-      CMAKE_ARGS -DBUILD_SHARED_LIBS=OFF -DBUILD_EXAMPLES=OFF -DBUILD_TESTING=OFF -DCRYPTO_BACKEND=OpenSSL -DOPENSSL_ROOT_DIR=${OPENSSL_BINARY_DIR}
+      CMAKE_ARGS -DBUILD_SHARED_LIBS=OFF -DBUILD_EXAMPLES=OFF -DBUILD_TESTING=OFF -DCRYPTO_BACKEND=OpenSSL -DOPENSSL_ROOT_DIR=${OPENSSL_BINARY_DIR} -DCMAKE_POSITION_INDEPENDENT_CODE=ON
       BUILD_IN_SOURCE NO
       TEST_COMMAND ""
       INSTALL_COMMAND ""