* In configure.ac, honour a preset SHELL_BASH when determining SHELL_BASH (so it
  can be passed in via autogen.input).

* For both toplevel "make" and per-module "cd $module && make", consistenly use
  the SHELL_BASH determined in configure.ac as the Make SHELL.  (By moving the
  setting to a new, common soleng/gbuild/shell.mk.  This also exports SHELL as
  an environment variable to sub-processes now; this exporting can probably be
  reverted if it is not what is actually wanted---but note that this exporting
  of an environment variable to sub-processes is unrelated to how nested
  invocations of $(MAKE) obtain their Make SHELL setting, see below.)

* Remove the gb_SHELL override feature from solenv/gbuild/gbuild.mk; for one, it
  is unclear how it was intended to interact with setting SHELL=@SHELL_BASH@ in
  the toplevel Makefile; for another, overriding SHELL can be done globally via
  setting SHELL_BASH in autogen.input now.

* Make treats SHELL specially, in that it never uses the SHELL environment
  variable to determine the Make SHELL variable.  Instead, if this Make
  invocation, or any outer Make invocation it is nested in via calls of $(MAKE),
  was called with a SHELL=... command line argument, then that value is used
  (and otherwise the default is hardwired as /bin/sh).  So, when calling nested
  invocations of $(MAKE) from the toplevel Makefile, pass down the current SHELL
  value via a SHELL=... command line argument via GMAKE_OPTIONS.  (And further
  nested invocations of $(MAKE) will then automatically inherit the SHELL value
  given on the outer invocation's command line.)

Change-Id: I67fa1b88e4e90d09456c1fcad2d082fdce3c019b