Fix VALID_CHARSETS test

The `VALID_CHARSETS` test assumes it is running in a dev virtual
environment, which is not necessarily true; for example, when running
`make hb-shape` to build HarfBuzz in CI. Now it only checks validity
when `CHARSET` is defined.

Makefile

@@ -47,9 +47,11 @@ INTERMEDIATE_FONTS = $(addprefix $(INTERMEDIATE_PREFIX),$(FONTS))
 SUBSET_PREFIX = subset-
 HB_PROGRAMS = hb-shape hb-view
 
-VALID_CHARSETS = $(shell PYTHONPATH="sources:$$PYTHONPATH" python3 -c 'import charsets; print(" ".join(charsets.Charset))')
-ifneq ($(strip $(filter-out $(VALID_CHARSETS),$(CHARSET))),)
-    $(error One or more invalid values: $(CHARSET); must be one of: $(VALID_CHARSETS))
+ifdef CHARSET
+    VALID_CHARSETS = $(shell PYTHONPATH="sources:$$PYTHONPATH" python3 -c 'import charsets; print(" ".join(charsets.Charset))')
+    ifneq ($(strip $(filter-out $(VALID_CHARSETS),$(CHARSET))),)
+        $(error One or more invalid values: $(CHARSET); must be one of: $(VALID_CHARSETS))
+    endif
 endif
 
 VALID_WEIGHTS = Regular Bold