Bug 1329737 - part 2 - turn CARGO_BUILD into a callable function; r=rillian

This change paves the way for injecting target- or host-specific
environment variables for a particular `cargo build` invocation.  Doing
this is not strictly necessary: all of our current `cargo build`
invocations use mostly target-specific environment
variables (e.g. PKG_CONFIG_ALLOW_CROSS).  But separating things out
makes the code notationally cleaner and also avoids weirdness when
target==host.

config/rules.mk

@@ -963,7 +963,15 @@ else
 environment_cleaner =
 endif
 
-CARGO_BUILD = env $(environment_cleaner) $(rustflags_override) \
+# This function is intended to be called by:
+#
+#   $(call CARGO_BUILD,EXTRA_ENV_VAR1=X EXTRA_ENV_VAR2=Y ...)
+#
+# but, given the idiosyncracies of make, can also be called without arguments:
+#
+#   $(call CARGO_BUILD)
+define CARGO_BUILD
+env $(environment_cleaner) $(rustflags_override) \
 	CARGO_TARGET_DIR=$(CARGO_TARGET_DIR) \
 	RUSTC=$(RUSTC) \
 	MOZ_DIST=$(ABS_DIST) \
@@ -972,7 +980,9 @@ CARGO_BUILD = env $(environment_cleaner) $(rustflags_override) \
 	PKG_CONFIG_ALLOW_CROSS=1 \
 	RUST_BACKTRACE=1 \
 	MOZ_TOPOBJDIR=$(topobjdir) \
+	$(1) \
 	$(CARGO) build $(cargo_build_flags)
+endef
 
 ifdef RUST_LIBRARY_FILE
 
@@ -987,7 +997,7 @@ endif
 # build.
 force-cargo-library-build:
 	$(REPORT_BUILD)
-	$(CARGO_BUILD) --lib $(cargo_target_flag) $(rust_features_flag)
+	$(call CARGO_BUILD) --lib $(cargo_target_flag) $(rust_features_flag)
 
 $(RUST_LIBRARY_FILE): force-cargo-library-build
 endif # RUST_LIBRARY_FILE
@@ -1000,7 +1010,7 @@ endif
 
 force-cargo-host-library-build:
 	$(REPORT_BUILD)
-	$(CARGO_BUILD) --lib $(cargo_host_flag) $(host_rust_features_flag)
+	$(call CARGO_BUILD) --lib $(cargo_host_flag) $(host_rust_features_flag)
 
 $(HOST_RUST_LIBRARY_FILE): force-cargo-host-library-build
 endif # HOST_RUST_LIBRARY_FILE
@@ -1008,14 +1018,14 @@ endif # HOST_RUST_LIBRARY_FILE
 ifdef RUST_PROGRAMS
 force-cargo-program-build:
 	$(REPORT_BUILD)
-	$(CARGO_BUILD) $(addprefix --bin ,$(RUST_CARGO_PROGRAMS)) $(cargo_target_flag)
+	$(call CARGO_BUILD) $(addprefix --bin ,$(RUST_CARGO_PROGRAMS)) $(cargo_target_flag)
 
 $(RUST_PROGRAMS): force-cargo-program-build
 endif # RUST_PROGRAMS
 ifdef HOST_RUST_PROGRAMS
 force-cargo-host-program-build:
 	$(REPORT_BUILD)
-	$(CARGO_BUILD) $(addprefix --bin ,$(HOST_RUST_CARGO_PROGRAMS)) $(cargo_host_flag)
+	$(call CARGO_BUILD) $(addprefix --bin ,$(HOST_RUST_CARGO_PROGRAMS)) $(cargo_host_flag)
 
 $(HOST_RUST_PROGRAMS): force-cargo-host-program-build
 endif # HOST_RUST_PROGRAMS