...which broke all the stock /bin shells to no longer pass through any DYLD_*
environment variables, so the DYLD_LIBRARY_PATH passed into the firebird Make
would not be passed to all the places that need it (to find the external/icu
libraries that some executables link against which are built and run as part of
building firebird).

What works with two little tweaks is to build your own bash and pass it to make
with SHELL=...:

* For one, there is an empbuild executable that uses system(...) to call another
  isql executable, where the latter needs DYLD_LIBRARY_PATH set, but which would
  not pass through the system(...) call (which implicityl uses /bin/sh).

* For another, it is still necessary to invoke Firebird's make with an explicit
  SHELL=$(SHELL), for reasons that are not entirely clear to me.  (There are
  some Makefile.in in Firebird's extern/ sub-tree that set "SHELL = @SHELL@" to
  configure's CONFIG_SHELL, unless overriden via an explicit command line
  arguemnt, but I don't think those are relevant here.)

Change-Id: I1e68faa898e758f09efb602d96fd6b35657e0480