Move 'pre-inst-env.in' to 'build-aux'.

* pre-inst-env.in: Move this...
* build-aux/pre-inst-env.in: ...here.
* configure.ac: Update reference.
* .gitignore: Do not ignore it, and remove a duplicate 'pre-inst-env'.

.gitignore

@@ -8,6 +8,7 @@
 /build-aux/*
 !/build-aux/git-version-gen
 !/build-aux/gitlab-ci.yml
+!/build-aux/pre-inst-env.in
 /config.log
 /config.status
 /configure
@@ -16,7 +17,6 @@
 /gash/config.scm
 /lcov.info
 /pre-inst-env
-/pre-inst-env
 /scripts/gash
 /tests/*.1
 /tests/*.2

configure.ac

@@ -21,7 +21,8 @@ AM_CONDITIONAL([HAVE_GENHTML], [test -n $GENHTML])
 
 AC_CONFIG_FILES([Makefile])
 AC_CONFIG_FILES([gash/config.scm])
-AC_CONFIG_FILES([pre-inst-env], [chmod +x pre-inst-env])
+AC_CONFIG_FILES([pre-inst-env:build-aux/pre-inst-env.in],
+                [chmod +x pre-inst-env])
 AC_CONFIG_FILES([tests/unit/config.scm])
 AC_CONFIG_FILES([tests/spec/Makefile])
 AC_CONFIG_FILES([tools/coverage], [chmod +x tools/coverage])