wrlib: generate automatically the list of symbols to keep from the API header

wrlib: generate automatically the list of symbols to keep from the API header

Instead of having to maintain manually the map file for LD with the list of
symbols that we want to keep in the WRaster library (the purpose is to hide
internal symbols), this patch implements a script that parses the public
API header and extracts automatically the list of names that are supposed
to be visible to the user.

The goal is to reduce possible human errors, like for example the function
RCombineAlpha that was forgotten from the list, yet still keep the map file
feature which is considered a good practice (it reduces the risk for name
clash and the risk of mis-use of internal stuff from the library).

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>