Avoid dependencies of build-deps on MacOS by pinning cc

Sadly we have to hack around cargo to make this work with
`-Zbuild-std`.

genbindings.sh

@@ -359,7 +359,20 @@ if [ "$HOST_PLATFORM" = "x86_64-unknown-linux-gnu" ]; then
 		LLVM_V=$(rustc +nightly --version --verbose | grep "LLVM version" | awk '{ print substr($3, 0, 2); }')
 		if [ -x "$(which clang-$LLVM_V)" ]; then
 			cargo +nightly clean
-			cargo +nightly rustc $CARGO_BUILD_ARGS -Zbuild-std=std,panic_abort --target x86_64-unknown-linux-gnu -v -- -Zsanitizer=memory -Zsanitizer-memory-track-origins -Cforce-frame-pointers=yes
+			# -Zbuild-std fails if we have any dependencies of build-deps, which
+			# cc added in 1.0.80, thus we pin back to 1.0.79 to avoid that.
+			pushd lightning-c-bindings
+			cargo update -p cc --precise "1.0.79" --verbose
+			popd
+			# Sadly, std also depends on cc, and we can't pin it in that tree
+			# directly. Instead, we have to delete the file out of the cargo
+			# registry and build --offline to avoid it using the latest version.
+			NEW_CC_DEP="$CARGO_HOME"
+			[ "$NEW_CC_DEP" = "" ] && NEW_CC_DEP="$HOME"
+			mv "$NEW_CC_DEP/.cargo/registry/cache/github.com-"*/cc-1.0.79.crate ./
+			rm -f "$NEW_CC_DEP/.cargo/registry/cache/github.com-"*/cc-*.crate
+			mv ./cc-1.0.79.crate "$NEW_CC_DEP/.cargo/registry/cache/github.com-"*/
+			cargo +nightly rustc --offline $CARGO_BUILD_ARGS -Zbuild-std=std,panic_abort --target x86_64-unknown-linux-gnu -v -- -Zsanitizer=memory -Zsanitizer-memory-track-origins -Cforce-frame-pointers=yes
 			mv target/x86_64-unknown-linux-gnu/debug/libldk.* target/debug/
 
 			# Sadly, std doesn't seem to compile into something that is memsan-safe as of Aug 2020,