Makefile: error on GENERATE_COMPILATION_DATABASE=yes failure

The "GENERATE_COMPILATION_DATABASE=yes" option added in
3821c38 (Makefile: add support for generating JSON compilation
database, 2020-09-03) positively asserts that you'd like to have a
compilation database generated.

If we can't generate the database because CC isn't clang, then we
shouldn't suppress the errors the compiler itself emits.

Doing so appears to have been done only because 3821c38
copy/pasted most of the code from the earlier
"COMPUTE_HEADER_DEPENDENCIES=auto" added in 111ee18 (Makefile: Use
computed header dependencies if the compiler supports it,
2011-08-18).

In the "COMPUTE_HEADER_DEPENDENCIES" we'd like to be more graceful
since we'll try to turn it on without the user asking, but as
"GENERATE_COMPILATION_DATABASE=yes" is purely opt-in we shouldn't
ignore STDERR on the user's behalf.

So let's stop suppressing STDERR, and call $(error) instead of
$(warning) if we fail here.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>

Makefile

@@ -1306,12 +1306,11 @@ ifeq ($(GENERATE_COMPILATION_DATABASE),yes)
 compdb_check = $(shell $(CC) $(ALL_CFLAGS) \
 	-Wno-pedantic \
 	-c -MJ /dev/null \
-	-x c /dev/null -o /dev/null 2>&1; \
+	-x c /dev/null -o /dev/null; \
 	echo $$?)
 ifneq ($(compdb_check),0)
-override GENERATE_COMPILATION_DATABASE = no
-$(warning GENERATE_COMPILATION_DATABASE is set to "yes", but your compiler does not \
-support generating compilation database entries)
+$(error Generating the compilation database failed. If your compiler does not \
+support generating compilation database entries, do not set GENERATE_COMPILATION_DATABASE=yes)
 endif
 else
 ifneq ($(GENERATE_COMPILATION_DATABASE),no)