Environment variables to configure tools for build machine

Hi,

The standard name to configure the compiler that compiles against the 
build machine (used e.g. to compile info/makedoc.c that runs during 
build even when cross-compiling) is CC_FOR_BUILD. However, texinfo’s 
configure.ac file uses BUILD_CC. The name CC_FOR_BUILD is much more 
common than BUILD_CC nowadays. E.g. there is a macro for CC_FOR_BUILD in 
autoconf-archive 
(https://www.gnu.org/software/autoconf-archive/ax_prog_cc_for_build.html).

Autoconf’s config.guess script renamed BUILD_CC to CC_FOR_BUILD in 1999 
[1]. Even at the time when cross-compiling support was added to texinfo, 
the config.guess script included in the texinfo sources was already 
using CC_FOR_BUILD. In theory, things could go wrong if only one of 
BUILD_CC or CC_FOR_BUILD are set when --build is not passed to configure.

For these two reasons, I suggest that texinfo should use CC_FOR_BUILD 
instead of BUILD_CC to compile info/makedoc.c etc. I can send a patch 
for this if nobody opposes. Also, I can send an additional patch adding 
CC_FOR_BUILD to `configure --help` (CC is already documented).

If this change is made, BUILD_CC could stay as a fallback for backwards 
compatibility. In this case, a warning could be printed.

(Texinfo also uses BUILD_AR and BUILD_RANLIB. Similar considerations 
apply to these.)

-Manuel

[1] 
https://gitweb.git.savannah.gnu.org/gitweb/?p=autoconf.git;a=commit;h=9c3f31ada2c39dd867e365913fbab00fbf2aa886