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Gnulib contains some header file overrides. This means that when building on systems with deficient header files in /usr/include/, it may create files named string.h, stdlib.h, stdint.h or similar in the build directory. In the other source directories of your package you will usually pass ‘-I’ options to the compiler, so that these Gnulib substitutes are visible and take precedence over the files in /usr/include/.
These Gnulib substitute header files rely on <config.h> being already included. Furthermore <config.h> must be the first include in every compilation unit. This means that to all your source files and likely also to all your tests source files you need to add an ‘#include <config.h>’ at the top. Which source files are affected? Exactly those whose compilation includes a ‘-I’ option that refers to the Gnulib library directory.
This is annoying, but inevitable: On many systems, <config.h> is
used to set system dependent flags (such as
_GNU_SOURCE on GNU systems),
and these flags have no effect after any system header file has been included.