#include in a C file

[ali hagigat]

4.13 Generating Prerequisites Automatically
To avoid this hassle, most modern C compilers can write these rules
for you, by looking
at the #include lines in the source files. Usually this is done with
the ‘-M’ option to the
compiler.
The practice we recommend for automatic prerequisite generation is to
have one makefile
corresponding to each source file.
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If compiler generates the necessary prerequisites automatically by -M
option why we need to have one makefile for each source file?