CHARSETALIASDIR in TESTS_ENVIRONMENT?

[Ben Pfaff]

The localcharset module has a dependency on "make install" that I
didn't realize until recently: until "make install" is run,
locale_charset cannot resolve character set aliases.  For PSPP at
least, this makes "make check" before "make install" fail, unless
CHARSETALIASDIR is explicitly set in the environment.

This makes me wonder whether localcharset should recommend that a
TESTS_ENVIRONMENT something like this should be used:
        TESTS_ENVIRONMENT = CHARSETALIASDIR='$(abs_top_builddir)/gl'
This fixes the problem for PSPP.

localcharset could go one step further by defining a substitution
variable for use in TESTS_ENVIRONMENT, e.g. for use like this:
        TEST_ENVIRONMENT = @LOCALCHARSET_TESTENV@
This is analogous to @FLOOR_LIBM@ etc.

Any thoughts?
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