Fix cross-compiling of ioctlsort

Fix cross-compiling of ioctlsort

Use the AX_PROG_CC_FOR_BUILD helper to set up build settings when cross
compiling.  This way ioctlsort uses the build tools all the time.

* configure.ac: Call AX_PROG_CC_FOR_BUILD.
* Makefile.am (ioctlsort_CC): Set to CC_FOR_BUILD.
(ioctlsort_CPPFLAGS): Change CPPFLAGS to CPPFLAGS_FOR_BUILD.
(ioctlsort_CFLAGS): Change CFLAGS to CFLAGS_FOR_BUILD.
(ioctlsort_LDFLAGS): Change LDFLAGS to LDFLAGS_FOR_BUILD.
(ioctlsort_LD): Delete.
(ioctlsort%): Change to ioctlsort%$(BUILD_EXEEXT).  Use ioctlsort_CFLAGS.
* m4/ax_prog_cc_for_build.m4: Import from the autoconf-archive package.