Fix -DEbml_DIR=/path/to/libebml in the README

[robUx4]

The Ebml_DIR option mention in the README.md doesn't exist and is ignored. I'm not sure what it the correct way to signal this (some combination of CMAKE_MODULE_PATH probably).

[mbunkus]

It's called EBML_DIR and works, but you often would rather use CMAKE_INSTALL_PREFIX.

For example, if you build & install libEBML like this:

cmake -DCMAKE_INSTALL_PREFIX=$HOME/opt/ebmlmatroska -DBUILD_SHARED_LIBS=no ..

you'll likely want to install libMatroska into the same directory, hence:

cmake -DCMAKE_INSTALL_PREFIX=$HOME/opt/ebmlmatroska -DBUILD_SHARED_LIBS=no ..

That suffices for both detection & setting the target directory.

However, this also compiles, but still installs in the default directory:

cmake -DEBML_DIR=$HOME/opt/ebmlmatroska/lib/cmake/EBML -DBUILD_SHARED_LIBS=no ..

[robUx4]

Indeed, it works when not setting CMAKE_INSTALL_PREFIX, otherwise it warns that the Ebml_DIR parameters has not been used.