In other external projects using libtool, we fix that by patching configure,
resetting hardcode_libdir_flag_spec[_CXX] at the end of the linux*) case block
that sets the Linux-specific value.  But here we run autoreconf in
ExternalProject_libassuan, so that patch in configure would be overwritten.  The
relevant code in configure comes from autoconf boilerplate, so we cannot just do
the same patch in configure.ac.  But we can reset hardcode_libdir_flag_spec
sufficiently late in configure.ac so that things still work as intended.

Disable tests that would build executabes linking against libgpgme.so, which in
turn links against the libassuan and libgpg-error libs, which would no longer be
found by the linker because of the dropped -rpath flags.  (Alternatives might
be to pass in LD_LIBRARY_PATH or to link with --allow-shlib-undefined.)

Change-Id: I7e37abf802d213347bd80383b7980d85cf0762d4
Reviewed-on: https://gerrit.libreoffice.org/50960Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>