gawk build hardcodes AR and ARFLAGS

[Jacob Burkholder]

Hi, I had trouble compiling gawk on a system that has
x86_64-pc-linux-gnu-ar but not ar.  I don't want to cross compile, I want
to override the ar used during the build with AR= like I do for CC=, but
gawk hardcodes AR and ARFLAGS in support/Makefile.in.  I fixed it with the
attached patch, other packages like m4 have a lot more logic for setting AR
in configure that I didn't duplicate.  Any chance to get this fixed?

0-configure
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