Fix check for 7za.

Check fails with MSYS2. bash ignores executable extension.
Merge pull request #234 from lazybiz/master

Fix check for 7za.
Revert "Fix check for 7za."

This reverts commit 1023274b99100f8d8da92a25411706dc6eaf40b2.
Merge pull request #235 from lazybiz/master

Revert last commit.