.INSTALL.in:
INSTALL.git:
Generate an error if the name of the current directory contains spaces.
.INSTALL.in:
Also generate an error if the current directory lacks a boehm_gc subdir,
indicating that it is not the result of unpacking a source distribution.
Mmakefile:
Make INSTALL executable after it is created from .INSTALL.in.
(INSTALL.git is already executable.)