Some demos based on CMake may search installed packages out of
Occlum toolchain's directory, this could cause some building
errors sometimes. By setting the $PKG_CONFIG_LIBDIR, we can make
CMake searching for packages in Occlum toolchain's directory only.
1. Disable zlib _explictly_ when building libcurl in remote attestaion demo.
The reason for this is that zlib may be implictly linked by the build system of
libcurl but not the zlib library is not copied into the Occlum FS image.
2. Use `make -j$(nproc)`
3. Fix typos
