Disable git-archive

Since configure uses git-version-gen which in turn depends
on "git describe" or  a .tarball-version file, raw archives
generated by "git archive" cannot be built.

* .gitattributes: New file.
* README-github_gitlab-autogenerated-archives.txt: Likewise.

.gitattributes

@@ -0,0 +1,2 @@
+/* export-ignore
+/README-github_gitlab-autogenerated-archives.txt -export-ignore

README-github_gitlab-autogenerated-archives.txt

@@ -0,0 +1,11 @@
+github/gitlab autogenerated archives are not supported
+======================================================
+
+This information applies to bundles automatically generated
+by github and gitlab.
+
+strace's program version is maintained using git-version-gen script, therefore,
+building strace depends on "git describe" (a full git clone is needed for this)
+or, alternatively, a .tarball-version file.  Neither github nor gitlab can
+provide the latter, they also do not allow to inject a .tarball-version file
+during export archiving.