build_cross_tools: export ACLOCAL to stop GCC from trying to use it.

This should fix the build of the crosstools on Haiku, CentOS, and
other platforms with versions of autofools too old or too new for
GCC 7.

build/scripts/build_cross_tools_gcc4

@@ -146,6 +146,11 @@ find "$binutilsSourceDir" "$gccSourceDir" -name \*.info -print0 | xargs -0 touch
 # version of autofools that GCC wants
 find "$binutilsSourceDir" "$gccSourceDir" \( -name configure -o -name \*.in \) -print0 | xargs -0 touch
 
+# export some variables to stop the subsequent configure/make from invoking
+# tools which we don't require the user to have installed (or do not require
+# a specific version of, which autofools does.)
+export ACLOCAL=:
+
 # create the object and installation directories for the cross compilation tools
 objDir="${installDir}-build"
 binutilsObjDir="$objDir/binutils"