libcxx: Don't remove the downloaded file eagerly

It's convenient during development.
Also, it can be useful for offline builds.

libs/libxx/.gitignore

@@ -1,2 +1,3 @@
 /uClibc++
 /libcxx
+/libcxx-*.src.tar.xz

libs/libxx/libcxx.defs

@@ -20,10 +20,11 @@
 
 VERSION=11.0.0
 
-libcxx:
+libcxx-$(VERSION).src.tar.xz:
 	$(Q) wget https://github.com/llvm/llvm-project/releases/download/llvmorg-$(VERSION)/libcxx-$(VERSION).src.tar.xz
+
+libcxx: libcxx-$(VERSION).src.tar.xz
 	$(Q) tar -xf libcxx-$(VERSION).src.tar.xz
-	$(Q) $(DELFILE) libcxx-$(VERSION).src.tar.xz
 	$(Q) mv libcxx-$(VERSION).src libcxx
 	$(Q) patch -p0 < 0001-libc-Fix-a-few-warnings.patch
 	$(Q) patch -p0 < 0001-libc-NFC-Fix-several-GCC-warnings-in-the-test-suite.patch
@@ -38,6 +39,7 @@ $(TOPDIR)/include/libcxx: libcxx
 dirlinks:: $(TOPDIR)/include/libcxx
 
 distclean::
+	$(Q) $(DELFILE) libcxx-$(VERSION).src.tar.xz
 	$(Q) $(DIRUNLINK) $(TOPDIR)/include/libcxx
 	$(call DELDIR, libcxx)