This little hack runs make recursively to generate include/loglevels.h
before running any other target but skip an explicit dependency on
loglevels.h in each rule. Therefore, you do not need to rebuild
every source file when compiling.
Using shell commands, a bitmask is generated for each file. Additionally,
a C header containing an array of loglevels and their files is created in
include/loglevels.h.
Michael Stapelberg