autotools: Rename "pregenerated" clean targets and make them .PHONY

This follows the automake conventions, see:
https://www.gnu.org/software/automake/manual/html_node/Clean.html

Makefile.am

@@ -218,10 +218,10 @@ precomp: $(PRECOMP)
 # e.g., after `make maintainer-clean`).
 BUILT_SOURCES = $(PRECOMP)
 
-maintainer-clean-local: clean-precomp
-
-clean-precomp:
+.PHONY: maintainer-clean-precomp
+maintainer-clean-precomp:
 	rm -f $(PRECOMP)
+maintainer-clean-local: maintainer-clean-precomp
 
 ### Pregenerated test vectors
 ### (see the comments in the previous section for detailed rationale)
@@ -234,10 +234,10 @@ testvectors: $(TESTVECTORS)
 
 BUILT_SOURCES += $(TESTVECTORS)
 
-maintainer-clean-local: clean-testvectors
-
-clean-testvectors:
+.PHONY: maintainer-clean-testvectors
+maintainer-clean-testvectors:
 	rm -f $(TESTVECTORS)
+maintainer-clean-local: maintainer-clean-testvectors
 
 ### Additional files to distribute
 EXTRA_DIST = autogen.sh CHANGELOG.md SECURITY.md