Use $(MAKE) and -C when running sub-make processes

Use $(MAKE) to run the same make even when it is not named 'make' or is
not first on the path.  Use -C for simplicity, since the Makefile
already relies on GNU make extensions.

Author: Kevin Pyle

Makefile

@@ -151,7 +151,7 @@ $(PLUGIN_DSO): $(PLUGIN_OBJECT_FILES) $(LIBGCC_C_API_SO)
 	    -lgcc-c-api -Lgcc-c-api -Wl,-rpath=$(GCCPLUGINS_DIR)
 
 $(LIBGCC_C_API_SO):
-	cd gcc-c-api && make libgcc-c-api.so CC="$(CC)" TARGET_GCC="$(TARGET_GCC)"
+	$(MAKE) -C gcc-c-api libgcc-c-api.so CC="$(CC)" TARGET_GCC="$(TARGET_GCC)"
 
 $(PLUGIN_OBJECT_GENERATED_FILES): CPPFLAGS+= $(if $(srcdir),-I$(srcdir))
 
@@ -172,7 +172,7 @@ clean:
 	$(RM) -r docs/_build
 	$(RM) -f gcc-with-$(PLUGIN_NAME).1 gcc-with-$(PLUGIN_NAME).1.gz
 	$(RM) -f print-gcc-version
-	cd gcc-c-api && make clean CC="$(CC)" TARGET_GCC="$(TARGET_GCC)"
+	$(MAKE) -C gcc-c-api clean CC="$(CC)" TARGET_GCC="$(TARGET_GCC)"
 	find tests -name "*.o" -delete
 
 autogenerated-config.h: $(addprefix $(srcdir),generate-config-h.py configbuilder.py)
@@ -205,7 +205,7 @@ mandir=/usr/share/man
 UpperPluginName = $(shell $(PYTHON) -c"print('$(PLUGIN_NAME)'.upper())")
 
 docs/_build/man/gcc-with-python.1: docs/gcc-with-python.rst
-	cd docs && $(MAKE) man
+	$(MAKE) -C docs man
 
 gcc-with-$(PLUGIN_NAME).1: docs/_build/man/gcc-with-python.1
 	# Fixup the generic manpage for this build:
@@ -224,7 +224,7 @@ man: gcc-with-$(PLUGIN_NAME).1.gz
 install: $(PLUGIN_DSO) gcc-with-$(PLUGIN_NAME).1.gz
 	mkdir -p $(DESTDIR)$(GCCPLUGINS_DIR)
 
-	cd gcc-c-api && $(MAKE) install
+	$(MAKE) -C gcc-c-api install
 
 	cp $(PLUGIN_DSO) $(DESTDIR)$(GCCPLUGINS_DIR)
 
@@ -290,7 +290,7 @@ show-ssa: plugin
 	$(INVOCATION_ENV_VARS) $(srcdir)./gcc-with-python examples/show-ssa.py test.c
 
 html: docs/tables-of-passes.rst docs/passes.svg
-	cd docs && $(MAKE) html
+	$(MAKE) -C docs $@
 
 # We commit this generated file to SCM to allow the docs to be built without
 # needing to build the plugin: