GeezerVison

On AIX, make(1) treats the value of SHELL as a literal executable
path rather than a command line. Since gh-100220 the top-level Makefile
sets SHELL = /bin/sh -e to make complex recipes fail-fast under GNU
make, but AIX then tries to execve("/bin/sh -e", ...) and the build
aborts immediately:

$ make
/bin/sh -e: not found
make: 1254-004 The error code from the last command is 1.

POSIX requires make to invoke the shell with -e by default; GNU
make does the same once a .POSIX: special target is declared (it sets
.SHELLFLAGS = -ec). This PR:

• adds .POSIX: as the first non-comment line in Makefile.pre.in;
• restores SHELL = /bin/sh so the value is a valid path on AIX;
• keeps the original fail-fast guarantee from make sharedinstall does not return failure if install commands fail #100220 on both POSIX and
  GNU make.

Verified locally with GNU Make 4.3 that a multi-command recipe
(echo a; false; echo b) aborts after false with .POSIX: set,
matching the prior SHELL = /bin/sh -e behavior.

• Issue: Invalid SHELL in Makefile for AIX builds? #149021