xetex build tests fail due to fontspec v2.6 defining ``\strong``

[jfbu]

latest update to fontspec defines \strong. This causes build failure for PDF output when latex_engine= 'xelatex'

! LaTeX Error: Command \strong already defined.
               Or name \end... illegal, see p.192 of the manual.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

fortunately, Sphinx anticipated this some time ago ;-)

** (sphinx) defining (legacy) text style macros without \sphinx prefix
** if clashes with packages, set latex_keep_old_macro_names=False in conf.py

Hence, default configuration for xelatex should do this latex_keep_old_macro_names=False. The problem will also arise with lualatex if it uses fontspec. Currently default setting of Sphinx with lualatex does not yet do it because it needed to pass tests with Ubuntu Precise which has TeXLive 2009 whose fontspec did not yet have good support for lualatex (see #3070 (comment))

Problem

• LaTeX PDF build fails

Procedure to reproduce the problem

• set latex_engine = 'xelatex' in conf.py and run (with Sphinx default font configuration for xelatex) with fully updated TeXLive 2016 install.

Error logs / results

as above

Expected results

latexpdf compiles

Reproducible project / your project

any project using xelatex

Environment info

• fully updated TeXLive 2016 (February 17, 2016)
• Sphinx version: 1.5.2

[jfbu]

fontspec link: wspr/fontspec@f3f4132

[jfbu]

the immediate problem is fixed at b0a5ec1 for 1.5.3 release. Discussion on future policy continues at #3429.