The dynamic generation of the linker version script for libnotmuch
exports has grown rather complicated.
Reverse the visibility control by hiding symbols by default using
-fvisibility=hidden, and explicitly exporting symbols in notmuch.h
using #pragma GCC visibility. (We could also use __attribute__
((visibility ("default"))) for each exported function, but the pragma
is more convenient.)
The above is not quite enough alone, as it would "leak" a number of
weak symbols from Xapian and C++ standard library. Combine it with a
small static version script that filters out everything except the
notmuch_* symbols that we explicitly exposed, and the C++ RTTI
typeinfo symbols for exception handling.
Finally, as the symbol hiding test can no longer look at the generated
symbol table, switch the test to parse the functions from notmuch.h.
