Fix various build issues when building on MinGW

[Joel Brobecker]

Hello,

Eli Zaretskii found that libctf does not build on MinGW, and sent
some patches on bugzilla:

    https://sourceware.org/bugzilla/show_bug.cgi?id=25155

He mentioned those patches in this mailing list
(https://sourceware.org/ml/binutils/2019-12/msg00277.html), but
since there has been no response, he's asked me to submit them
on his behalf. Here they are (except for the first one, which is
from me, and pretty obvious).

Note that these patches are holding the GDB 9 release, so a prompt
review would be appreciated.

 * [RFA 1/3] libctf: Regenerate Makefile.in and acinclude.m4
 * [RFA 2/3] libctf: Add configure check for asprintf (for MinGW)
 * [RFA 3/3] libctf: compilation failure on MinGW due to missing errno

These patches were tested by rebuilding on GNU/Linux. I do not have
access to a Windows machine with the compiler using the version of
MinGW Eli uses, so Eli, could you double-check that everything still
works? In particular, I had to re-generate the configure and config.h.in
files.

OK to push to master?

Thank you,
-- 
Joel