INSTALL_BASE Not Propagated to DIR entries

[theory]

Noticed while building Locale::TextDomain:

curl -O https://cpan.metacpan.org/authors/id/G/GU/GUIDO/libintl-perl-1.29.tar.gz
tar zxf libintl-perl-1.29.tar.gz
cd libintl-perl-1.29
perl -i -pe 's/#DIR/DIR/' Makefile.PL
perl Makefile.PL INSTALL_BASE=/tmp/install/here

Note that it won't bother running the Makefile in the gettext_xs directory if it doesn't think it'll build, so the second-to-last line of this block uncomments a line in the Makefile.PL to force it to operate on the subdirectory. Good enough to demonstrate the issue:

> grep ^INSTALL_BASE Makefile  
INSTALL_BASE = /tmp/install/here
> grep ^INSTALL_BASE gettext_xs/Makefile

:-( Latest version of EU::MM:

 perl -MExtUtils::MakeMaker -E 'say ExtUtils::MakeMaker->VERSION'
7.34