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20.5 How Do I ‘#define’ Installation Directories?
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     My program needs library files, installed in ‘datadir’ and
     similar.  If I use

          AC_DEFINE_UNQUOTED([DATADIR], [$datadir],
            [Define to the read-only architecture-independent
             data directory.])

     I get

          #define DATADIR "${prefix}/share"

   As already explained, this behavior is on purpose, mandated by the
GNU Coding Standards, see *note Installation Directory Variables::.
There are several means to achieve a similar goal:

   − Do not use ‘AC_DEFINE’ but use your makefile to pass the actual
     value of ‘datadir’ via compilation flags.  *Note Installation
     Directory Variables::, for the details.

   − This solution can be simplified when compiling a program: you may
     either extend the ‘CPPFLAGS’:

          CPPFLAGS = -DDATADIR='"$(datadir)"' @CPPFLAGS@

     If you are using Automake, you should use ‘AM_CPPFLAGS’ instead:

          AM_CPPFLAGS = -DDATADIR='"$(datadir)"'

     Alternatively, create a dedicated header file:

          DISTCLEANFILES = myprog-paths.h
          myprog-paths.h: Makefile
                  echo '#define DATADIR "$(datadir)"' >$@

     The Gnulib module ‘configmake’ provides such a header with all the
     standard directory variables defined, *note (gnulib)configmake::.

   − Use ‘AC_DEFINE’ but have ‘configure’ compute the literal value of
     ‘datadir’ and others.  Many people have wrapped macros to automate
     this task; for an example, see the macro ‘AC_DEFINE_DIR’ from the
     Autoconf Macro Archive
     (https://www.gnu.org/software/autoconf-archive/).

     This solution does not conform to the GNU Coding Standards.

   − Note that all the previous solutions hard wire the absolute name of
     these directories in the executables, which is not a good property.
     You may try to compute the names relative to ‘prefix’, and try to
     find ‘prefix’ at runtime, this way your package is relocatable.