contrib: add helper to invoke CMake

contrib/src/main.mak

@@ -181,6 +181,9 @@ UNPACK_DIR = $(basename $(basename $(notdir $<)))
 APPLY = (cd $(UNPACK_DIR) && patch -p1) <
 MOVE = mv $(UNPACK_DIR) $@ && touch $@
 
+CMAKE = cmake . -DCMAKE_TOOLCHAIN_FILE=$(abspath toolchain.cmake) \
+		-DCMAKE_INSTALL_PREFIX=$(PREFIX)
+
 #
 # Per-package build rules
 #

contrib/src/mpcdec/rules.mak

@@ -38,10 +38,8 @@ endif
 	touch $@
 
 .mpcdec: musepack toolchain.cmake
-	cd $< && cmake . \
-		-DCMAKE_TOOLCHAIN_FILE=../toolchain.cmake \
-		-DCMAKE_INSTALL_PREFIX="$(PREFIX)"
-	cd $< && make
+	cd $< && $(CMAKE) .
+	cd $< && $(MAKE)
 	mkdir -p -- "$(PREFIX)/lib"
 ifdef HAVE_WIN32
 	cd $< && cp libmpcdec/libmpcdec_static.a "$(PREFIX)/lib/libmpcdec.a"