As described in the attached thread, the build system needs to be taught to
rebuild config_keys.inc whenever svn_config.h is touched, otherwise developers
who add new config file options/sections (SVN_CONFIG_* macros) will have to
manually run gen-make.py to avoid spurious spell-checking warnings (see
getopt_tests.py 9, getopt_config_option()).
This only affects developers' workflow; it shouldn't affect end-user binaries as
those would typically be generated from clean trees.