Some large packages using FreeType have to use a broken (deficient)
/bin/sh. The configure scripts (as generated by Autoconf) are
clever enough to find a better shell and put that one into the
environment variable CONFIG_SHELL. If that environment variable is
already set the script skips the test and assumes to be already
running under a good shell.
* builds/unix/detect.mk: Honour CONFIG_SHELL.
* builds/unix/unix-def.in (SHELL): Define.
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