build: Fix out of tree builds

The '$<' variable is not guarantee to work in ordinary make rules by
non GNU make implementations. In addition if it is supported it will
be replaced by the actual source pathname (in VPATH mode), which is not
the relative name we need on installation.

Ref: https://www.gnu.org/software/autoconf/manual/autoconf.html#g_t_0024_003c-in-Ordinary-Make-Rules
Reported-by: David Seifert <soap@gentoo.org>

man/Makefile.am

@@ -41,7 +41,7 @@ md2_FUNCS = \
 	$(nil)
 
 $(md2_FUNCS): md2.3
-	$(LN_S) -f $< $(DESTDIR)$(man3dir)/$@.3
+	$(LN_S) -f md2.3 $(DESTDIR)$(man3dir)/$@.3
 
 md4_FUNCS = \
 	MD4Transform \
@@ -56,7 +56,7 @@ md4_FUNCS = \
 	$(nil)
 
 $(md4_FUNCS): md4.3
-	$(LN_S) -f $< $(DESTDIR)$(man3dir)/$@.3
+	$(LN_S) -f md4.3 $(DESTDIR)$(man3dir)/$@.3
 
 md5_FUNCS = \
 	MD5Transform \
@@ -71,7 +71,7 @@ md5_FUNCS = \
 	$(nil)
 
 $(md5_FUNCS): md5.3
-	$(LN_S) -f $< $(DESTDIR)$(man3dir)/$@.3
+	$(LN_S) -f md5.3 $(DESTDIR)$(man3dir)/$@.3
 
 rmd160_FUNCS = \
 	RMD160Transform \
@@ -86,7 +86,7 @@ rmd160_FUNCS = \
 	$(nil)
 
 $(rmd160_FUNCS): rmd160.3
-	$(LN_S) -f $< $(DESTDIR)$(man3dir)/$@.3
+	$(LN_S) -f rmd160.3 $(DESTDIR)$(man3dir)/$@.3
 
 sha1_FUNCS = \
 	SHA1Transform \
@@ -101,7 +101,7 @@ sha1_FUNCS = \
 	$(nil)
 
 $(sha1_FUNCS): sha1.3
-	$(LN_S) -f $< $(DESTDIR)$(man3dir)/$@.3
+	$(LN_S) -f sha1.3 $(DESTDIR)$(man3dir)/$@.3
 
 sha2_FUNCS = \
 	SHA256Init \
@@ -134,7 +134,7 @@ sha2_FUNCS = \
 	$(nil)
 
 $(sha2_FUNCS): sha2.3
-	$(LN_S) -f $< $(DESTDIR)$(man3dir)/$@.3
+	$(LN_S) -f sha2.3 $(DESTDIR)$(man3dir)/$@.3
 
 digest_FUNCS = \
 	$(md2_FUNCS) \