depends: force a new host id string if FORCE_USE_SYSTEM_CLANG is in use

This should be caught by the differing clang --version outputs, but because we
haven't yet extracted our pinned clang, the system one is actually used for the
version check. That's not a problem because bumping our pinned clang will
cause a rebuild of everything anyway.

depends/Makefile

@@ -125,6 +125,11 @@ $(host_arch)_$(host_os)_id_string+=$(shell $(host_CXX) --version 2>/dev/null)
 $(host_arch)_$(host_os)_id_string+=$(shell $(host_RANLIB) --version 2>/dev/null)
 $(host_arch)_$(host_os)_id_string+=$(shell $(host_STRIP) --version 2>/dev/null)
 
+ifneq ($(strip $(FORCE_USE_SYSTEM_CLANG)),)
+build_id_string+=system_clang
+$(host_arch)_$(host_os)_id_string+=system_clang
+endif
+
 qrencode_packages_$(NO_QR) = $(qrencode_packages)
 
 qt_packages_$(NO_QT) = $(qt_packages) $(qt_$(host_os)_packages) $(qt_$(host_arch)_$(host_os)_packages) $(qrencode_packages_)