Makefile: remove hard bindgen(1) dep

Makefile

@@ -28,13 +28,18 @@ MANDIR != test $(PREFIX_N) = / && printf '/usr/share/man\n' \
 SYSEXITS != printf '\043include <sysexits.h>\n' | cpp -M - | tr ' ' '\n' \
 	| sed -n 's/sysexits\.h//p' || printf 'include\n'
 
+BINDGEN ?= bindgen
+BINDGENFLAGS ?= --fit-macro-constant-types \
+	--default-macro-constant-type unsigned --use-core --formatter=none
+
 CC ?= cc
+CFLAGS += -I$(SYSEXITS)
+
 RUSTC ?= rustc
 RUSTFLAGS += --extern delimit=build/o/libdelimit.rlib \
 	--extern getopt=build/o/libgetopt.rlib \
 	--extern strerror=build/o/libstrerror.rlib \
 	--extern sysexits=build/o/libsysexits.rlib
-CFLAGS += -I$(SYSEXITS)
 
 # testing requires the absolute path to the bin directory set
 BIN = build/bin
@@ -71,15 +76,15 @@ TESTS != printf '%s\n' "$(TESTFILES)" | xargs -n1 basename \
 include $(TESTFILES)
 
 .PHONY: test
-test: all $(TESTS) /tmp/delimit /tmp/getopt
+test: all $(TESTS) build/test/delimit build/test/getopt
 	@echo $(TESTS)
-	/tmp/delimit
-	/tmp/getopt
+	build/test/delimit
+	build/test/getopt
 
-/tmp/delimit: src/libdelimit.rs
+build/test/delimit: src/libdelimit.rs
 	$(RUSTC) --test -o $@ src/libdelimit.rs
 
-/tmp/getopt: src/libgetopt.rs
+build/test/getopt: src/libgetopt.rs
 	$(RUSTC) --test -o $@ src/libgetopt.rs
 
 .PHONY: docs
@@ -109,8 +114,8 @@ build/o/libstrerror.rlib: build src/libstrerror.rs
 		src/libstrerror.rs
 
 build/o/libsysexits.rlib: build/include/sysexits.h
-	bindgen --fit-macro-constant-types --default-macro-constant-type unsigned --use-core --formatter=none \
-		build/include/sysexits.h | $(RUSTC) $(RUSTFLAGS) --crate-type lib -o $@ -
+	$(BINDGEN) $(BINDGENFLAGS) build/include/sysexits.h \
+		| $(RUSTC) $(RUSTFLAGS) --crate-type lib -o $@ -
 
 # bandage solution until bindgen(1) gets stdin support
 build/include/sysexits.h: build $(SYSEXITS)sysexits.h