contrib: Allow cmake make invocation to communicate with the jobserver

Since make can't ensure $(CMAKEBUILD) will invoke $(MAKE) itself, the sub-make process wasn't able to communicate with the jobserver, causing jobs to run with a single job instead of however much are available.

contrib/src/srt/rules.mak

@@ -33,5 +33,5 @@ srt: srt-$(SRT_VERSION).tar.gz .sum-srt
 .srt: srt toolchain.cmake
 	cd $< && $(HOSTVARS_PIC) CFLAGS="$(SRT_CFLAGS)" CXXFLAGS="$(SRT_CXXFLAGS)" $(CMAKE) \
 		-DENABLE_SHARED=OFF -DUSE_GNUTLS=ON -DENABLE_CXX11=OFF -DCMAKE_INSTALL_LIBDIR=lib -DCMAKE_INSTALL_BINDIR=bin -DCMAKE_INSTALL_INCLUDEDIR=include
-	cd $< && $(CMAKEBUILD) . --target install
+	+$(CMAKEBUILD) $< --target install
 	touch $@