cygwin build fix for master, gawk-5.1-stable

[Tom Gray]

Hi All,

Surprise! More mpfr related maintenance.

gawk-5.1-stable and master both fail to build in Cygwin.

In gawk-5.1-stable,
        extension/rwarray requires linking with mpfr library

In Master
        extension/rwarray requires linking with mpfr library
        extension/testext requires linking with mpfr library
        several files, flag BOOL conflicts with windows "typedef int BOOL;" 
needed for pc/popen.c

In the attached diff I changed the flag BOOL to BOOLF to distinguish it from 
the windows BOOL.
Link changes are in extensions/Makefile.am

Tested in Cygwin, Msys2, Fedora 35.
Make check is happy in all cases.

Tom

gawk_cygwin_buil.diff
Description: gawk_cygwin_buil.diff