follow standard configure behavior for cross compile prefix

the standard configure interface, which our configure script tries to
implement, identifies cross compiling (build != host) and searches for
the properly-prefixed cross tools. our script was not doing that,
forcing users to explicitly provide either CC or a CROSS_COMPILE tool
prefix, and the more common choice, just providing CC, was incomplete
because the Makefile would still invoke the native ar and ranlib
programs. this happened to work when building on ELF-based systems
with GNU binutils, but could easily fail when cross-compiling from
dissimilar systems.

like before, and like the standard configure behavior, an explicit CC
or CROSS_COMPILE variable on the command line or in the environment
overrides the automatic prefixing.

configure

@@ -25,6 +25,7 @@ Fine tuning of the installation directories:
 System types:
   --target=TARGET         configure to run on target TARGET [detected]
   --host=HOST             same as --target
+  --build=BUILD           build system type; used only to infer cross-compiling
 
 Optional features:
   --enable-optimize=...   optimize listed components for speed over size [auto]
@@ -129,6 +130,7 @@ includedir='$(prefix)/include'
 syslibdir='/lib'
 tools=
 tool_libs=
+build=
 target=
 optimize=auto
 debug=no
@@ -172,6 +174,7 @@ case "$arg" in
 --disable-gcc-wrapper|--enable-gcc-wrapper=no) wrapper=no ;;
 --enable-*|--disable-*|--with-*|--without-*|--*dir=*|--build=*) ;;
 --host=*|--target=*) target=${arg#*=} ;;
+--build=*) build=${arg#*=} ;;
 -* ) echo "$0: unknown option $arg" ;;
 CC=*) CC=${arg#*=} ;;
 CFLAGS=*) CFLAGS=${arg#*=} ;;
@@ -180,7 +183,7 @@ LDFLAGS=*) LDFLAGS=${arg#*=} ;;
 CROSS_COMPILE=*) CROSS_COMPILE=${arg#*=} ;;
 LIBCC=*) LIBCC=${arg#*=} ;;
 *=*) ;;
-*) target=$arg ;;
+*) build=$arg ; target=$arg ;;
 esac
 done
 
@@ -213,6 +216,15 @@ done
 set +C
 trap 'rm "$tmpc"' EXIT INT QUIT TERM HUP
 
+#
+# Check whether we are cross-compiling, and set a default
+# CROSS_COMPILE prefix if none was provided.
+#
+test "$target" && \
+test "$target" != "$build" && \
+test -z "$CROSS_COMPILE" && \
+CROSS_COMPILE="$target-"
+
 #
 # Find a C compiler to use
 #