rust: rebuilds std when building fuzzers

so as to have MSAN working

configure.ac

@@ -475,6 +475,7 @@
     AC_ARG_ENABLE(fuzztargets,
         AS_HELP_STRING([--enable-fuzztargets], [Enable fuzz targets]),[enable_fuzztargets=$enableval],[enable_fuzztargets=no])
     AM_CONDITIONAL([BUILD_FUZZTARGETS], [test "x$enable_fuzztargets" = "xyes"])
+    AM_CONDITIONAL([RUST_BUILD_STD], [test "x$enable_fuzztargets" = "xyes" && echo $rust_compiler_version | grep -q nightly])
     AC_PROG_CXX
     AS_IF([test "x$enable_fuzztargets" = "xyes"], [
         AC_DEFINE([FUZZ], [1], [Fuzz targets are enabled])

rust/Makefile.am

@@ -7,9 +7,13 @@ if HAVE_CARGO_VENDOR
 EXTRA_DIST +=	vendor
 endif
 
+if RUST_BUILD_STD
+RELEASE = -Z build-std
+else
 if !DEBUG
 RELEASE = --release
 endif
+endif
 
 if HAVE_LUA
 RUST_FEATURES +=	lua $(LUA_INT8)