Fix quoting in the add_to_path Makefile macro.

The previous quoting caused "make -C src/bin check" to ignore, rather
than add to, any LD_LIBRARY_PATH content from the environment.
Back-patch to 9.4, where the macro was introduced.

src/Makefile.global.in

@@ -302,7 +302,7 @@ PROVE_FLAGS = --verbose
 
 # prepend to path if already set, else just set it
 define add_to_path
-$(1)='$(if $($(1)),$(2):$$$(1),$(2))'
+$(1)="$(if $($(1)),$(2):$$$(1),$(2))"
 endef
 
 # platform-specific environment variable to set shared library path