gendoc infra: disable pdf manual generation if xsltproc is buggy

gendoc infra: disable pdf manual generation if xsltproc is buggy

The PDF manual generation reaches the default xsltproc's template
recursion limit when processing the target package list; this makes the
PDF manual generation fail [1-3].

This limit can be raised with the '--maxvars' option. Unfortunately,
this option is not correctly handled in the latest xsltproc/libxslt
release (1.1.28), but this bug is already fixed in the libxslt
repository [4].

This patch disables the PDF manual generation (makes it warn with a
meaningful error message) when the xsltproc program found in the PATH
does not support the --maxvars option.
So, one can still generate the PDF manual if he/she extends PATH with
the location of a working xsltproc, by running:

  $ PATH=/path/to/custom-xsltproc/bin:${PATH} make manual-pdf

[1] http://lists.busybox.net/pipermail/buildroot/2014-August/104390.html
[2] http://lists.busybox.net/pipermail/buildroot/2014-August/104418.html
[3] http://lists.busybox.net/pipermail/buildroot/2014-August/104421.html
[4] https://gitorious.org/libxslt/libxslt/commit/5af7ad745323004984287e48b42712e7305de35c

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
[yann.morin.1998@free.fr: move the assignment block out of GENDOC_INNER, no
 need to retest for each type of each document: it's always the same answer;
 make it a warning as per Thomas DS. suggestion]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>