This avoids failing the build like

```
CMake Error at /usr/share/cmake-3.13/Modules/CMakeTestCCompiler.cmake:52 (message):
  The C compiler

    "/usr/lib/ccache/cc"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: /build/lurch-0.6.9/lib/axc/lib/libsignal-protocol-c/build/CMakeFiles/CMakeTmp

    Run Build Command:"/usr/bin/make" "cmTC_ff1ea/fast"
    make[2]: Entering directory '/build/lurch-0.6.9/lib/axc/lib/libsignal-protocol-c/build/CMakeFiles/CMakeTmp'
    /usr/bin/make -f CMakeFiles/cmTC_ff1ea.dir/build.make CMakeFiles/cmTC_ff1ea.dir/build
    make[3]: Entering directory '/build/lurch-0.6.9/lib/axc/lib/libsignal-protocol-c/build/CMakeFiles/CMakeTmp'
    Building C object CMakeFiles/cmTC_ff1ea.dir/testCCompiler.c.o
    /usr/lib/ccache/cc   -fPIC    -o CMakeFiles/cmTC_ff1ea.dir/testCCompiler.c.o   -c /build/lurch-0.6.9/lib/axc/lib/libsignal-protocol-c/build/CMakeFiles/CMakeTmp/testCCompiler.c
    Linking C executable cmTC_ff1ea
    /usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_ff1ea.dir/link.txt --verbose=1
    /usr/lib/ccache/cc -fPIC   -Wl,-z,relro -Wl,-z,now -ldl -lm -lsqlite3 -lmxml -lpthread -lglib-2.0 -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/libpurple -L/usr/lib/purple-2 -lxml2 -lgcrypt -ljabber -Wl,-rpath,/usr/lib/purple-2  -rdynamic CMakeFiles/cmTC_ff1ea.dir/testCCompiler.c.o  -o cmTC_ff1ea
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `xmlnode_set_prefix'
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `purple_conv_chat_get_topic'
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `purple_request_field_list_set_multi_select'
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `purple_imgstore_add_with_id'
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `purple_media_error'
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `purple_request_fields_new'
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `purple_prpl_got_user_idle'
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `purple_account_set_password'
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `purple_conv_chat_user_set_flags'
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `purple_strdup_withhtml'
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `purple_cipher_context_new_by_name'
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `_purple_network_set_common_socket_flags'
    /usr/bin/ld: /usr/lib/purple-2/libjabber.so: undefined reference to `purple_xfer_set_cancel_recv_fnc'
    ...
```