fix a 'make dist' bug

[Bruno Haible]

The generated tarball is only usable on the same platform on which it was
generated.

How to reproduce: I use a system that has libtextstyle not installed in /usr
but instead in a directory that I indicate through the --prefix option and
CPPFLAGS and LDFLAGS variables. I can build poke (from the git repository),
but when I run "make distcheck", it fails:

/bin/bash ../libtool  --tag=CC   --mode=compile gcc -ftrapv -DHAVE_CONFIG_H -I. 
-I../../../libpoke -I../poke  -I../gl-libpoke -I../../../gl-libpoke 
-I../../../common -DPKL_DEBUG 
-DPKGDATADIR=\"/POKE/poke/build-64/poke-0.90/_inst/share/poke\" 
-DPKGINFODIR=\"/POKE/poke/build-64/poke-0.90/_inst/share/info\" 
-DLOCALEDIR=\"/POKE/poke/build-64/poke-0.90/_inst/share/locale\" 
-fvisibility=hidden -DBUILDING_LIBPOKE -DJITTER_DISPATCH_DIRECT_THREADING=1 -I 
/POKE/poke/build-64/poke-0.90/jitter -I 
/POKE/poke/build-64/poke-0.90/_build/sub/jitter -I 
/POKE/poke/build-64/poke-0.90/jitter/machine/x86_64 -I 
/POKE/poke/build-64/poke-0.90/_inst/include  -Wall 
-I/arch/x86_64-linux-gnu/gnu-inst-gc/7.4.2/include  -g -O2 -MT 
libpoke_la-pvm.lo -MD -MP -MF .deps/libpoke_la-pvm.Tpo -c -o libpoke_la-pvm.lo 
`test -f 'pvm.c' || echo '../../../libpoke/'`pvm.c
libtool: compile:  gcc -ftrapv -DHAVE_CONFIG_H -I. -I../../../libpoke -I../poke 
-I../gl-libpoke -I../../../gl-libpoke -I../../../common -DPKL_DEBUG 
-DPKGDATADIR=\"/POKE/poke/build-64/poke-0.90/_inst/share/poke\" 
-DPKGINFODIR=\"/POKE/poke/build-64/poke-0.90/_inst/share/info\" 
-DLOCALEDIR=\"/POKE/poke/build-64/poke-0.90/_inst/share/locale\" 
-fvisibility=hidden -DBUILDING_LIBPOKE -DJITTER_DISPATCH_DIRECT_THREADING=1 -I 
/POKE/poke/build-64/poke-0.90/jitter -I 
/POKE/poke/build-64/poke-0.90/_build/sub/jitter -I 
/POKE/poke/build-64/poke-0.90/jitter/machine/x86_64 -I 
/POKE/poke/build-64/poke-0.90/_inst/include -Wall 
-I/arch/x86_64-linux-gnu/gnu-inst-gc/7.4.2/include -g -O2 -MT libpoke_la-pvm.lo 
-MD -MP -MF .deps/libpoke_la-pvm.Tpo -c ../../../libpoke/pvm.c  -fPIC -DPIC -o 
.libs/libpoke_la-pvm.o
In file included from 
/POKE/poke/build-64/poke-0.90/jitter/jitter/jitter-instruction.h:34:0,
                 from ../../libpoke/pvm.jitter:170,
                 from ../../../libpoke/pvm-program.h:22,
                 from ../../../libpoke/pvm.c:31:
/POKE/poke/build-64/poke-0.90/jitter/jitter/jitter-print.h:31:71: fatal error: 
textstyle.h: No such file or directory
compilation terminated.
make[3]: *** [Makefile:2386: libpoke_la-pvm.lo] Error 1
make[3]: Leaving directory '/POKE/poke/build-64/poke-0.90/_build/sub/libpoke'
make[2]: *** [Makefile:2102: all] Error 2
make[2]: Leaving directory '/POKE/poke/build-64/poke-0.90/_build/sub/libpoke'
make[1]: *** [Makefile:2032: all-recursive] Error 1
make[1]: Leaving directory '/POKE/poke/build-64/poke-0.90/_build/sub'
make: *** [Makefile:2250: distcheck] Fehler 1

What's going on? The sub-build run by Automake's 'distcheck' target does not
use my --prefix, CPPFLAGS, LDFLAGS arguments. So it runs in an environment
that does not have libtextstyle. But it uses the file
jitter/jitter/jitter-config.h which was generated by the outer build and which
contains
  #define JITTER_HAVE_LIBTEXTSTYLE 1
And that's because this file has been added to the tarball. But config.h files
are platform dependent and therefore should never be added to the tarball.

Here is a proposed fix. With it, I can run "make distcheck", and the tarball
no longer contains jitter-config.h. The fix follows the recommended pattern
from https://www.gnu.org/software/automake/manual/html_node/The-dist-Hook.html .

0001-Don-t-package-jitter-jitter-config.h-into-the-tarbal.patch
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