Using $< in a non-suffix rule context is a GNU make idiom.
This bug was introduced in 7.41.0.

--- docs/libcurl/Makefile.am.orig Tue Feb 24 22:49:02 2015
+++ docs/libcurl/Makefile.am Tue Mar 17 22:13:19 2015
@@ -94,7 +94,7 @@
 SUFFIXES = .3 .html
 
 libcurl-symbols.3: $(srcdir)/symbols-in-versions $(srcdir)/mksymbolsmanpage.pl
- perl $(srcdir)/mksymbolsmanpage.pl < $< > $@
+ perl $(srcdir)/mksymbolsmanpage.pl < $(srcdir)/symbols-in-versions > $@
 
 html: $(HTMLPAGES)
         cd opts; make html

-- 
Christian "naddy" Weisgerber                          naddy_at_mips.inka.de
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