fix: [Makefile] Use $(MAKE) for parallel builds

Using $(MAKE) enables the parallel builds and removes jobserver
unavailable warning.

Refer:

https://stackoverflow.com/q/50510278

Author: manid2

Makefile

@@ -16,22 +16,22 @@ FORMAT_SUB_DIRS:=$(foreach subdir,$(SUB_DIRS),format-$(subdir))
 LINT_SUB_DIRS:=$(foreach subdir,$(SUB_DIRS),lint-$(subdir))
 
 $(SUB_DIRS): venv
-	make -C $@
+	$(MAKE) -C $@
 
 $(TESTS_SUB_DIRS): venv
-	make -C $(@:tests-%=%) tests
+	$(MAKE) -C $(@:tests-%=%) tests
 
 $(CHECK_SUB_DIRS): $(TESTS_SUB_DIRS)
-	make -C $(@:check-%=%) check
+	$(MAKE) -C $(@:check-%=%) check
 
 $(CLEAN_SUB_DIRS):
-	make -C $(@:clean-%=%) clean
+	$(MAKE) -C $(@:clean-%=%) clean
 
 $(FORMAT_SUB_DIRS):
-	make -C $(@:format-%=%) format
+	$(MAKE) -C $(@:format-%=%) format
 
 $(LINT_SUB_DIRS):
-	make -C $(@:lint-%=%) lint
+	$(MAKE) -C $(@:lint-%=%) lint
 
 # ============================================================================
 # Setup venv for python and docs
@@ -59,10 +59,10 @@ CPP_SRCS_LINT:=$(addsuffix -lint,$(CPP_BINS))
 CPP_ALL:=$(CPP_BINS) $(CPP_BINS_CLEAN) $(CPP_SRCS_FORMAT) $(CPP_SRCS_LINT)
 
 $(CPP_BINS_CHECK):
-	$(Q)make -C cpp $(@:cpp/%=%)
+	$(Q)$(MAKE) -C cpp $(@:cpp/%=%)
 
 $(CPP_ALL):
-	make -C cpp $(@:cpp/%=%)
+	$(MAKE) -C cpp $(@:cpp/%=%)
 
 # ============================================================================
 # Top level targets

cpp/Makefile

@@ -30,7 +30,7 @@ SRCS_FORMAT:=$(addsuffix -format,$(BINS))
 SRCS_LINT:=$(addsuffix -lint,$(BINS))
 
 $(BINS_CHECK):
-	$(Q)make $(@:-check=)
+	$(Q)$(MAKE) $(@:-check=)
 	$(Q)./$(BUILD_DIR)/$(@:-check=)
 
 $(BINS_CLEAN):

docs/Makefile

@@ -22,7 +22,7 @@ html: venv
 venv: $(VENV_DONE)
 
 $(VENV_DONE):
-	make -C ../ venv
+	$(MAKE) -C ../ venv
 
 # ============================================================================
 # Top level targets

python3/Makefile

@@ -15,7 +15,7 @@ MODULES:=$(sort $(dir $(shell $(GIT_LS) -- '*/__init__.py')))
 venv: $(VENV_DONE)
 
 $(VENV_DONE):
-	make -C ../ venv
+	$(MAKE) -C ../ venv
 
 flake8: venv
 	$(VENV_ACTIVATE) flake8 $(MODULES)