build-sys: Add "release build" flag, use for symbol versioning

build-sys: Add "release build" flag, use for symbol versioning

I was trying to do a release and move the symbols from `-devel.sym` into
`-release.sym`, but it turns out that at least GNU binutils `ld` treats an empty
version script as a syntax error.  Fix this by adding a "release build"
flag, and only include `-devel` in non-release builds.

This would also make it easier to inject that flag into our `.pc` and
`ostree-version.h` and `ostree --version` metadata, but I didn't do that yet.

EDIT: Turns out a simpler fix is just to add an empty section. However I kept
this commit since it's a useful sanity check for whether we should include
`-devel.sym` in builds, and we may want to inject the metadata later.

Closes: #942
Approved by: jlebon