WIP: BUILD_LATEX: latex export support

[dhimmel]

Refs #249

[dhimmel]

There's a problem creating the pdf in the xelatex command in 78678e9 (rebased to 2cdb61f):

[7Error: /syntaxerror in /----nostringval----
Operand stack:

Execution stack:
   %interp_exit   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %stopped_push   --nostringval--   --nostringval--   --nostringval--   false   1   %stopped_push   1992   1   3   %oparray_pop   1991   1   3   %oparray_pop   1979   1   3   %oparray_pop   1833   1   3   %oparray_pop   --nostringval--   %errorexec_pop   .runexec2   --nostringval--   --nostringval--   --nostringval--   2   %stopped_push
Dictionary stack:
   --dict:739/1123(ro)(G)--   --dict:0/20(G)--   --dict:75/200(L)--
Current allocation mode is local
GPL Ghostscript 9.27: Unrecoverable error, exit code 1

xdvipdfmx:fatal: pdf_ref_obj(): passed invalid object.

No output PDF file written.
xelatex: fwrite: Broken pipe