autoconf: use AC_CHECK_TOOL for as and ld
Some platforms use the GNU target triplet as a prefix to the toolchain
utilities (e.g. x86_64-pc-linux-gnu-as) to allow the use of alternative
toolchains, cross-compiling, etc.
The Gentoo Linux distribution has a mode of building packages
(-native-symlinks) where the toolchain utilities only exist as their
prefixed names (e.g. 'as' does not exist, but 'x86_64_pc-linux-gnu-as'
does). This results in configure failing to locate the tools when using
AC_CHECK_PROGS. (Gentoo uses the --host and --build configure
parameters to specify the prefix names for the tools).
Replace AC_CHECK_PROGS with AC_CHECK_TOOL for the toolchain related
commands 'as' and 'ld'.
AC_CHECK_TOOL works like AC_CHECK_PROGS but it will also look for
the program with a prefix (specified by using configure's --host
parameter).
Note: libtool.m4 runs AC_CHECK_TOOL for ar.
Change-Id: I8005c765d213b7d1d6292a7dd80f10a3d0e2ec68
Reviewed-on: https://gerrit.openafs.org/14544
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>