portability: don't use absolute paths to headers

There's no /usr/include on Mojave.

* python/vcsn_cxx.cc: here.
4 jobs for master in 10 minutes and 50 seconds (queued for 1 second)
Name Stage Failure
failed
debian:gcc:debug Test
bison++: did not find any file in ../lib/vcsn/rat/parse.dir
make: *** [Makefile:4814: lib/vcsn/rat/parse.stamp] Error 1
make: *** Waiting for unfinished jobs....
Uploading artifacts...
build/*.log: found 1 matching files
WARNING: build/*/*.log: no matching files
WARNING: build/*/*/*.log: no matching files
Uploading artifacts to coordinator... ok
id=75043 responseStatus=201 Created token=kLtur8sA
ERROR: Job failed: exit code 1
failed
debian:clang:speed Test
bison++: did not find any file in ../lib/vcsn/rat/parse.dir
make: *** [Makefile:4814: lib/vcsn/rat/parse.stamp] Error 1
make: *** Waiting for unfinished jobs....
Uploading artifacts...
build/*.log: found 1 matching files
WARNING: build/*/*.log: no matching files
WARNING: build/*/*/*.log: no matching files
Uploading artifacts to coordinator... ok
id=75044 responseStatus=201 Created token=YEA4kh6r
ERROR: Job failed: exit code 1
failed
archlinux:clang:debug Test
warning: ignoring: configuration([{'arg': 'key', 'class': 'std::string', 'const': 'const ', 'default': None, 'type': 'const std::string&'}]) -> std::string
warning: ignoring: set_format([{'arg': 'out', 'class': 'std::ostream', 'const': None, 'default': None, 'type': 'std::ostream&'}, {'arg': 'format', 'class': 'std::string', 'const': 'const ', 'default': None, 'type': 'const std::string&'}]) -> void
warning: ignoring: get_format([{'arg': 'out', 'class': 'std::ostream', 'const': None, 'default': None, 'type': 'std::ostream&'}]) -> std::string
Uploading artifacts...
build/*.log: found 1 matching files
WARNING: build/*/*.log: no matching files
WARNING: build/*/*/*.log: no matching files
Uploading artifacts to coordinator... ok
id=75042 responseStatus=201 Created token=czPsH98F
ERROR: Job failed: exit code 1
failed
archlinux:gcc:speed Test
bison++: cannot convert ../lib/vcsn/rat/parse.dir/parse.xml report to parse.html
make: *** [Makefile:4974: lib/vcsn/rat/parse.stamp] Error 1
make: *** Waiting for unfinished jobs....
Uploading artifacts...
build/*.log: found 1 matching files
WARNING: build/*/*.log: no matching files
WARNING: build/*/*/*.log: no matching files
Uploading artifacts to coordinator... ok
id=75041 responseStatus=201 Created token=asCiXJE2
ERROR: Job failed: exit code 1