build: avoid racy failure of "make -jN install"

build: avoid racy failure of "make -jN install"

Installing with -j2 or greater could result in a failure like
  /bin/sh: line 29: /P/bin/install: Permission denied
when /P/bin/install specifies your PATH-selected install program.
This would arise because we're using "install" to install all
man/*.1 files, and that command would run concurrently with the one
that installs "/P/bin/install" itself.  We would run this command:
"src/ginstall src/ginstall /P/bin/install", and it would result
in intervals during which the destination file does not exist,
is empty or incomplete and not executable.  We addressed this
problem long ago for installation of actual binaries by telling the
installation rules to use our just-built bin/ginstall (only when not
cross-compiling) rather than the PATH-resolved "install" program.
This change is to do the same for those .1 files.
* src/local.mk (INSTALL): Override automake's default of something
like "INSTALL = /P/bin/install -c".
(INSTALL_PROGRAM): Now that we set INSTALL, there is no longer any
need to set this derived variable.  Its default definition,
"INSTALL_PROGRAM = ${INSTALL}" does what we require.
Improved by Eric Blake.