Use curly braces for variable expansion in makefiles.

Building doc when `--disable-asciidoc' is broken on NetBSD. From 398012 in linus/makefile-var-exp:

* doc/Makefile.am: Change $(VAR:MOD) to ${VAR:MOD} -- make(1) on
NetBSD substitutes '$(:x)' to 'x' rather than the empty string.  This
bites us in doc/ when configured with `--disable-asciidoc'.  Curly
braces should work in all implementations of make(1) but this patch
changes only the places where we use the VAR:MOD expansion.

https://gitweb.torproject.org/linus/tor.git/shortlog/refs/heads/makefile-var-exp