ICU-5628 Fix using install-sh in out-of-source builds

[dg0yt]

Cf. microsoft/vcpkg#53279:
icu's configure.ac uses AC_PROG_INSTALL which tries to find a fast binary install and uses a slow install-sh script as fallback. Via @INSTALL@, the result is baked into icudefs.mk in the toplevel build directory.
When the fallback is used, icu prepends $(top_srcdir) for in-source builds (i.e. ${srcdir} = "."). (I assume this was meant as a fix for ICU-525 which is the same error for in-source builds.)
For out-of-source builds, nothing is prepended, and so the value of INSTALL is valid only for the toplevel build dir. This leads to errors when it is actually used in subdirs.
This change prepends $(top_builddir) instead, so that a valid path is construct for every possible case. Prepending is scoped to situations where srcpath is relative.

To reproduce the original issue, rename /usr/bin/install and similar candidates before running ../path/to/icu/source/configure.

The change from this PR is in configure.ac. configure is generated from that file.

Checklist

• Required: Issue filed: ICU-5628
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• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
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[dg0yt]

Unchanged guard, and matching an effect in AC_PROG_INSTALL.

[dg0yt]

Guard based on srcdir, similar to original code. I believe srcdir makes sense because it is used to construct the search path - and thus becomes part of the value - for install-sh.
Now any relative path instead of just ., and now prepending top_builddir instead of top_srcdir.

[dg0yt]

Disclaimer: I did not test this with in-source builds.