build failure on Solaris 11.4

[Bruno Haible]

The continuous integration today reported test failures on Solaris 11.4:
All 60 install-info tests fail.

The problem is that 'defs' contains:
diff="diff  -u "
and the 60 tests then reference ${diff} in the first position of a command
line.

This appears to be a problem with the shell: When I change the shebang line
#!/bin/sh -x
of a test to
#!/usr/bin/bash -x
the test succeeds. But changing SHELL to /usr/bin/bash in Makefile and in
defs does *not* fix it.

The workaround is easy: Instead of using a variable that includes the 'diff'
command name, use a variable for the options only. The attached patch fixes
the 60 failures.

Bruno

0001-build-Fix-failure-of-all-install-info-tests-on-Solar.patch
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