Simplify flags/... generation

pkg-config can take multiple arguments and will combine all
dependencies, so no need for any loops and so many intermediate steps.

Makefile

@@ -100,30 +100,24 @@ ocaml-freestanding.pc: ocaml-freestanding.pc.in Makeconf
 	    -e 's!@@PKG_CONFIG_EXTRA_LIBS@@!$(PKG_CONFIG_EXTRA_LIBS)!' \
 	    ocaml-freestanding.pc.in > $@
 
-flags/libs.tmp:
+flags/libs.tmp: flags/libs.tmp.in
 	opam config subst $@
 
 flags/libs: flags/libs.tmp Makeconf
-	sed -e 's!@@PKG_CONFIG_EXTRA_LIBS@@!$(PKG_CONFIG_EXTRA_LIBS)!' \
-	    flags/libs.tmp > flags/libs.tmp2
-	for PKG in $(PKG_CONFIG_DEPS); do \
-		echo " " >> flags/libs.tmp2;\
-		env PKG_CONFIG_PATH=$(shell opam config var prefix)/lib/pkgconfig pkg-config $$PKG --libs >> flags/libs.tmp2;\
-	done
-	echo "("`cat flags/libs.tmp2`")" > flags/libs
-
-flags/cflags.tmp:
+	env PKG_CONFIG_PATH="$(shell opam config var prefix)/lib/pkgconfig" \
+	    pkg-config $(PKG_CONFIG_DEPS) --libs >> flags/libs.tmp
+	echo "("`cat flags/libs.tmp`")" \
+	    | sed -e 's!@@PKG_CONFIG_EXTRA_LIBS@@!$(PKG_CONFIG_EXTRA_LIBS)!' \
+	    > flags/libs
+
+flags/cflags.tmp: flags/cflags.tmp.in
 	opam config subst $@
 
 flags/cflags: flags/cflags.tmp Makeconf
-	for PKG in $(PKG_CONFIG_DEPS); do \
-		echo " " >> flags/cflags.tmp;\
-		env PKG_CONFIG_PATH=$(shell opam config var prefix)/lib/pkgconfig pkg-config $$PKG --cflags >> flags/cflags.tmp;\
-	done
+	env PKG_CONFIG_PATH="$(shell opam config var prefix)/lib/pkgconfig" \
+	    pkg-config $(PKG_CONFIG_DEPS) --cflags >> flags/cflags.tmp
 	echo "("`cat flags/cflags.tmp`")" > flags/cflags
 
-
-
 install: all
 	./install.sh
 
@@ -132,5 +126,5 @@ uninstall:
 
 clean:
 	rm -rf build config Makeconf ocaml-freestanding.pc
-	rm -rf flags/libs flags/libs.tmp flags/libs.tmp2
+	rm -rf flags/libs flags/libs.tmp
 	rm -rf flags/cflags flags/cflags.tmp