Makefile: build libgit-rs and libgit-sys serially

"make -JN" with INCLUDE_LIBGIT_RS enabled causes cargo lock warnings
and can trigger ld errors during the build.

The build errors are caused by two inner "make" invocations getting
triggered concurrently: once inside of libgit-sys and another inside of
libgit-rs.

Make libgit-rs depend on libgit-sys so that "make" prevents them
from running concurrently. Apply the same logic to the test invocations.
Use cargo's "--manifest-path" option instead of "cd" in the recipes.

Signed-off-by: David Aguilar <[e-mail]>
Acked-by: Kyle Lippincott <[e-mail]>
Signed-off-by: Junio C Hamano <[e-mail]>

Makefile

···
 	$(MAKE) -C t/ unit-tests
 
 .PHONY: libgit-sys libgit-rs
-libgit-sys libgit-rs:
-	$(QUIET)(\
-		cd contrib/$@ && \
-		cargo build \
-	)
+libgit-sys:
+	$(QUIET)cargo build --manifest-path contrib/libgit-sys/Cargo.toml
+libgit-rs: libgit-sys
+	$(QUIET)cargo build --manifest-path contrib/libgit-rs/Cargo.toml
 ifdef INCLUDE_LIBGIT_RS
-all:: libgit-sys libgit-rs
+all:: libgit-rs
 endif
 
 LIBGIT_PUB_OBJS += contrib/libgit-sys/public_symbol_export.o

t/Makefile

···
 
 .PHONY: libgit-sys-test libgit-rs-test
 libgit-sys-test:
-	$(QUIET)(\
-		cd ../contrib/libgit-sys && \
-		cargo test \
-	)
-libgit-rs-test:
-	$(QUIET)(\
-		cd ../contrib/libgit-rs && \
-		cargo test \
-	)
+	$(QUIET)cargo test --manifest-path ../contrib/libgit-sys/Cargo.toml
+libgit-rs-test: libgit-sys-test
+	$(QUIET)cargo test --manifest-path ../contrib/libgit-rs/Cargo.toml
 ifdef INCLUDE_LIBGIT_RS
-all:: libgit-sys-test libgit-rs-test
+all:: libgit-rs-test
 endif