rust.inc: start passing *FLAGS vars to compile

When rust's build system builds llvm itself, it uses these flags. Avoid
them interfering with other things by cleaning their usage out of the
platform cfg mk file.

recipes-devtools/rust/rust.inc

@@ -320,6 +320,15 @@ def rust_gen_mk_cfg(d, thing):
     i = open(d.getVar('S', True) + '/mk/cfg/' + rust_base_sys + '.mk', 'r')
 
     r = subprocess.call(['sed',
+        # CFLAGS, LDFLAGS, CXXFLAGS, CPPFLAGS are used by rust's build for a
+        # wide range of targets (not just HOST). Yocto's settings for them will
+        # be inappropriate, avoid having random targets try to use them, we'll
+        # add as needed.
+        '-e', 's/$(CFLAGS)//',
+        '-e', 's/$(CXXFLAGS)//',
+        '-e', 's/$(CPPFLAGS)//',
+        '-e', 's/$(LDFLAGS)//',
+
         # update all triplets to the new one
         '-e', 's/{}/{}/g'.format(rust_base_sys, sys),
 
@@ -377,14 +386,6 @@ do_configure () {
 
     # FIXME: target_prefix vs prefix, see cross.bbclass
 
-    # CFLAGS, LDFLAGS, CXXFLAGS, CPPFLAGS are used by rust's build for a
-    # wide range of targets (not just HOST). Yocto's settings for them will
-    # be inappropriate, avoid using.
-    unset CFLAGS
-    unset LDFLAGS
-    unset CXXFLAGS
-    unset CPPFLAGS
-
     # FIXME: this path to rustc (via `which rustc`) may not be quite right in the case
     # where we're reinstalling the compiler. May want to try for a real
     # path based on bitbake vars