src/meson.build: Switch shared_library => library

This is the recommendation from meson documentation. The default option is still to create a shared library but doing it this way gives users an opportunity to create either a shared, static or both type of libraries by setting up the meson build appropriately.

Here is the result with the default setup:

$ meson setup . _build
$ meson compile -C _build
$ find _build | egrep "(\.so|\.a)$"

_build/subprojects/munit/libmunit.so
_build/test/libunittest.a
_build/test/libeierpecken.a
_build/src/libei.so
_build/src/liboeffis.so
_build/src/libutil.a
_build/src/libeis.so

And here it is with the overridden flag:

$ meson setup . _build  --default-library=static
$ meson compile -C _build
$ find _build | egrep "(\.so|\.a)$"

_build/subprojects/munit/libmunit.a
_build/test/libunittest.a
_build/test/libeierpecken.a
_build/src/libei.a
_build/src/libeis.a
_build/src/liboeffis.a
_build/src/libutil.a