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.

Makefile

@@ -19,6 +19,10 b' TESTFLAGS ?= $(shell echo $$HGTESTFLAGS)'
 # 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 b' all: build doc'
 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