This possibility got inadvertently removed in the big config refactor
(3ad20d7), which made building a shared library from the bundled source
distribution (as is done by e.g. vcpkg) impossible.
The old name was not really appropriate; this attribute should only be
used on variable declarations, while the definition of the same variable
needs to be tagged with 'WEPOLL_INTERNAL'.
* Configuration is now done with files, instead of through command line
switches specified in CMakeLists.txt.
* Link tests with a static library instead of compiling individual
wepoll source files as part of the test executable. This should make
the CI run significantly faster.
