depends: Use default macos clang compiler

Suggested by Cory Fields <cory-nospam-@coryfields.com>
https://github.com/bitcoin/bitcoin/pull/16367#issuecomment-595393546
as alternate workaround for problem described
https://github.com/bitcoin/bitcoin/pull/16367#issuecomment-594600985

depends/builders/darwin.mk

@@ -1,5 +1,5 @@
-build_darwin_CC:=$(shell xcrun -f clang)
-build_darwin_CXX:=$(shell xcrun -f clang++)
+build_darwin_CC:=clang
+build_darwin_CXX:=clang++
 build_darwin_AR:=$(shell xcrun -f ar)
 build_darwin_RANLIB:=$(shell xcrun -f ranlib)
 build_darwin_STRIP:=$(shell xcrun -f strip)