Don't hang the build forever if ttf-libertine.sty is not installed

I believe this bug is not present in the current master. This is what I get:

[2/106] Generating gp4-hdl.pdf
FAILED: doc/gp4-hdl.pdf
cd /home/whitequark/Work/openfpga/build/doc && pdflatex -halt-on-error /home/whitequark/Work/openfpga/doc/gp4-hdl.tex > /dev/null && pdflatex -halt-on-error /home/whitequark/Work/openfpga/doc/gp4-hdl.tex > /dev/null

and the logfile ends with:

(/usr/share/texlive/texmf-dist/tex/generic/pgf/frontendlayer/tikz/libraries/tik
zlibrarytopaths.code.tex
File: tikzlibrarytopaths.code.tex 2008/06/17 v3.0.1a (rcs-revision 1.2)
)))

! LaTeX Error: File `libertine.sty' not found.

Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: sty)

Enter file name:
! Emergency stop.
<read *>

l.14 \usepackage
                {zi4}^^M
Here is how much of TeX's memory you used:
 15304 strings out of 494945
 289244 string characters out of 6181032
 348833 words of memory out of 5000000
 18389 multiletter control sequences out of 15000+600000
 4709 words of font info for 16 fonts, out of 8000000 for 9000
 14 hyphenation exceptions out of 8191
 55i,0n,54p,450b,66s stack positions out of 5000i,500n,10000p,200000b,80000s

!  ==> Fatal error occurred, no output PDF file produced!

azonenberg / openfpga

Don't hang the build forever if ttf-libertine.sty is not installed #60