gherkin-c: Allow user to choose whether to build a static or a dynamic lib
gherkin-c: Allow user to choose whether to build a static or a dynamic lib

It should be the users choice whether they want a static or a dynamic
library. In general, CMake allow the user to specify that via the
BUILD_SHARED_LIBS variable. However when explicitly specifying
`SHARED` in the `add_library` call the `BUILD_SHARED_LIBS` is ignore.

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