Builds the set of available fonts for the given configuration. Note that
any changes to the configuration after this call (through
FcConfigParseAndLoad or
FcConfigParseAndLoadFromMemory) have indeterminate
effects. (On the other hand, application fonts can still be modified
through FcConfigAppFontAddFile,
FcConfigAppFontAddDir and
FcConfigAppFontClear). Returns FcFalse if this operation
runs out of memory. If config is NULL, the current
configuration is used.