Currently we explicitly construct a list of include directories and
compile definitions to pass to the wrapping parsers from module
variables.  With CMake 3.1 and above we can use `file(GENERATE)` and the
`$<TARGET_PROPERTY:...>` generator expression to get the
`INCLUDE_DIRECTORIES` and `COMPILE_DEFINITIONS` of the wrapped module
library at generate time from CMake.  This can actually be done in a few
earlier versions of CMake too but we need 3.1 to have a generator
expression in the `add_custom_command` `DEPENDS` option.

This approach avoids use of manually constructed lists of preprocessor
flags and always ensures that the wrapping parsers get the same
preprocessor flags as the compiler will get when compiling the module
library.  It will also allow us to get preprocessor flags through CMake
usage requirements (e.g. target_include_directories and
target_compile_definitions) for third-party system libraries.