keywords in comments not ignored

A user reported on the Debian bug tracker that deheader also recognizes keywords in comments and suggests the inclusion of the corresponding headers, even though the function is not actually used in the code.

Example:

% cat > x.c
int main(int argc, char *argv[]) { return 0; /* log10(SIZE_MAX) */ }
% deheader x.c 
deheader: in x.c, log10() portability requires <math.h>.
deheader: saw 1 files, 0 includes, 0 removable
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