Build libcurl as part of the 'make third' rule...

...without adding it to the standard list of required third party
dependencies.
Load the usecmap directives recursively for builtin CMaps.

Fixes bug introduced in commit 1679c1e7a89ae62260fd84ce55c6bef376c6e6ba:

    Optimize UniXXX CMap files.
Fix 695247: Use exponential realloc pattern and qsort for huge paths.

Grow the edge list using an exponential realloc pattern.
Use qsort for huge paths and only fall back to the simple
shell sort for small paths.
Fix 695222: Treat non-breaking space (U+00A0) as white space for search.