Remove Boost requirement

Newer Poppler versions require this, or for it to be explicitly disabled.

We are using the Splash output rendering for adding bitmaps,
but let's disable the requirement for now, to avoid the extra dependency.

Dockerfile

@@ -109,6 +109,7 @@ RUN --mount=type=cache,src=/tmp/ccache,target=/tmp/ccache,id=ccache,from=cacheba
           -DENABLE_GLIB:BOOL=OFF \
           -DENABLE_CPP:BOOL=OFF \
           -DENABLE_UTILS:BOOL=OFF \
+          -DENABLE_BOOST:BOOL=OFF \
           -DBUILD_SHARED_LIBS:BOOL=OFF \
           -DENABLE_UNSTABLE_API_ABI_HEADERS:BOOL=ON \
           -DCMAKE_BUILD_TYPE:STRING=release \