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