build: Proper quoting for var printing targets

Previously, if the value contained syntax that was meaningful to make,
the printing would fail. Quoting properly avoids this.

Makefile.am

@@ -4,7 +4,7 @@
 
 # Pattern rule to print variables, e.g. make print-top_srcdir
 print-%:
-	@echo $* = $($*)
+	@echo '$*' = '$($*)'
 
 ACLOCAL_AMFLAGS = -I build-aux/m4
 SUBDIRS = src

depends/Makefile

@@ -2,7 +2,7 @@
 
 # Pattern rule to print variables, e.g. make print-top_srcdir
 print-%:
-	@echo $* = $($*)
+	@echo '$*' = '$($*)'
 
 # When invoking a sub-make, keep only the command line variable definitions
 # matching the pattern in the filter function.

src/Makefile.am

@@ -4,7 +4,7 @@
 
 # Pattern rule to print variables, e.g. make print-top_srcdir
 print-%:
-	@echo $* = $($*)
+	@echo '$*' = '$($*)'
 
 DIST_SUBDIRS = secp256k1 univalue