make: alter how we compute compiler flags for setup.py stable

make: alter how we compute compiler flags for setup.py

This is portable between BSD and GNU make. I'm not thrilled with how
it worked out, but it's portable and solves the problem.

--- a/Makefile	Sat Apr 23 16:11:05 2016 +0900
+++ b/Makefile	Thu Apr 21 10:05:14 2016 -0400
@@ -19,6 +19,10 @@
 # Set this to e.g. "mingw32" to use a non-default compiler.
 COMPILER=
 
+COMPILERFLAG_tmp_ =
+COMPILERFLAG_tmp_${COMPILER} ?= -c $(COMPILER)
+COMPILERFLAG=${COMPILERFLAG_tmp_${COMPILER}}
+
 help:
 	@echo 'Commonly used make targets:'
 	@echo '  all          - build program and documentation'
@@ -43,16 +47,16 @@
 local:
 	$(PYTHON) setup.py $(PURE) \
 	  build_py -c -d . \
-	  build_ext $(COMPILER:%=-c %) -i \
-	  build_hgexe $(COMPILER:%=-c %) -i \
+	  build_ext $(COMPILERFLAG) -i \
+	  build_hgexe $(COMPILERFLAG) -i \
 	  build_mo
 	env HGRCPATH= $(PYTHON) hg version
 
 build:
-	$(PYTHON) setup.py $(PURE) build $(COMPILER:%=-c %)
+	$(PYTHON) setup.py $(PURE) build $(COMPILERFLAG)
 
 wheel:
-	FORCE_SETUPTOOLS=1 $(PYTHON) setup.py $(PURE) bdist_wheel $(COMPILER:%=-c %)
+	FORCE_SETUPTOOLS=1 $(PYTHON) setup.py $(PURE) bdist_wheel $(COMPILERFLAG)
 
 doc:
 	$(MAKE) -C doc