CARGO and RUSTC are standard variables that may be pre-set by the
build environment (e.g., Portage's cargo eclass) to point to specific
tool paths.  Use HAS_CARGO/HAS_RUSTC as boolean checks so that these
presence checks do not collide with the caller-supplied paths.

Based on the equivalent fix applied to 3.0.x by Matt Jolly <kangie@gentoo.org>.

--- a/configure.ac
+++ b/configure.ac
@@ -120,10 +120,10 @@
     [rust_vendor_sources=""])
 AC_SUBST([rust_vendor_sources])
 if test "$enable_rust_offline" = yes; then
-    AC_CHECK_PROG(CARGO, [cargo], [yes], [no])
-    AC_CHECK_PROG(RUSTC, [rustc], [yes], [no])
+    AC_CHECK_PROG(HAS_CARGO, [cargo], [yes], [no])
+    AC_CHECK_PROG(HAS_RUSTC, [rustc], [yes], [no])

-    AS_IF([test "$CARGO" != "yes" -o "$RUSTC" != "yes"], [
+    AS_IF([test "$HAS_CARGO" != "yes" -o "$HAS_RUSTC" != "yes"], [
       AC_MSG_FAILURE("Rust based plugins cannot be built cargo=$CARGO rustc=$RUSTC")
     ])
 fi