Now users don't need to override installation prefix for MinGW programs and
'configure' scripts use more compatible triplet 'x86_64-w64-mingw32`
automatically instead of 'x86_64-pc-mingw64`.
MSYSTEM now defaults to MSYS when not specified, and there is no
different behavior anymore between empty value and MSYS, for example
/opt/bin was not being added to path when not setting MSYSTEM. This
removes the need for explicitly setting the variable, since a default
value is now implied.
* New script /etc/msystem providing information associated with the
current MSYSTEM. This is sourced by /etc/profile but can be used by
non-login scripts for getting information such as the current
compiler architecture or mount point.
* New script /usr/bin/shell for switching between shells without
restarting MSYS2. This can be used in interactive sessions but also
from shell scripts for switching the MSYSTEM without restarting bash
or spawning a subshell.
* Allow /etc/profile to be sourced multiple times without continuously
appending stuff to PATH or losing track of the original Windows TEMP.
Also implemented for /etc/profile.d/perlbin.sh, so perl path is not
repeatedly appended.
