• Thomas Haller's avatar
    build: create "config-extra.h" header instead of passing directory variables via CFLAGS · a75ab799
    Thomas Haller authored
    1) the command line gets shorter. I frequently run `make V=1` to see
       the command line arguments for the compiler, and there is a lot
       of noise.
    2) define each of these variables at one place. This makes it easy
       to verify that for all compilation units, a particular
       define has the same value. Previously that was not obvious or
       even not the case (see commit e5d1a713
       and commit d63cf1ef).
       The point is to avoid redundancy.
    3) not all compilation units need all defines. In fact, most modules
       would only need a few of these defines. We aimed to pass the necessary
       minium of defines to each compilation unit, but that was non-obvious
       to get right and often we set a define that wasn't used. See for example
       "src_settings_plugins_ibft_cppflags" which needlessly had "-DSYSCONFDIR".
       This question is now entirely avoided by just defining all variables in
       a header. We don't care to find the minimum, because every component
       gets anyway all defines from the header.
    4) this also avoids the situation, where a module that previously did
       not use a particular define gets modified to require it. Previously,
       that would have required to identify the missing define, and add
       it to the CFLAGS of the complation unit. Since every compilation
       now includes "config-extra.h", all defines are available everywhere.
    5) the fact that each define is now available in all compilation units
       could be perceived as a downside. But it isn't, because these defines
       should have a unique name and one specific value. Defining the same
       name with different values, or refer to the same value by different
       names is a bug, not a desirable feature. Since these defines should
       be unique accross the entire tree, there is no problem in providing
       them to every compilation unit.
    6) the reason why we generate "config-extra.h" this way, instead of using
       AC_DEFINE() in configure.ac, is due to the particular handling of
       autoconf for directory variables. See [1].
       With meson, it would be trivial to put them into "config.h.meson".
       While that is not easy with autoconf, the "config-extra.h" workaround
       seems still preferable to me.
    [1] https://www.gnu.org/software/autoconf/manual/autoconf-2.63/html_node/Installation-Directory-Variables.html
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