build: fix crossruby build by handling empty `ac_abs_builddir`HEADmaster

`ac_abs_builddir` can be empty when the build is top-level (not
subdirs, and Ruby is usually the case). In such case, the MINIRUBY is
expanded to `$RUBY -I -r'$(arch)-fake'`, which interprets `-r$(arch)-fake`
as an argument to `-I` option. This led to:
- Not loading the fake config file
- Then not setting `CROSS_COMPILING` constant during extmk.rb execution
- Then misusing cross-compiled `./miniruby` instead of baseruby to generate
  files used in exts.

This commit fixes the issue by handling the empty `ac_abs_builddir` case
properly.

0 files changed, 0 insertions, 0 deletions