The DocBook 1.74.* had problems with different XSLT processors.

The DocBook 1.74.* had problems with different XSLT processors.
Add instructions to release instructions and make Makefiles more
flexible to allow 'make check' to *actually* be run with something
other than xsltproc. Thanks to Sorin Ristache for reporting this
issue.

   Because of problems with the 1.74.* releases, it is now
   recommended that "make check" be run multiple times with
   different XSLT processors.

   One recommended way to invoke "make check" is the following:

     rm -f DOCBOOK-BUILD.LOG && \
     . ~/docbk.sh && \
     make check 2>&1 \
       XSLTENGINE=xsltproc \
       | tee DOCBOOK-BUILD.LOG && \
     $DOCBOOK_SVN/buildtools/build-check DOCBOOK-BUILD.LOG

   After running with "xsltproc", you should run it again with
   "saxon":
     rm -f DOCBOOK-BUILD.LOG && \
     . ~/docbk.sh && \
     make check 2>&1 \
       XSLTENGINE=saxon \
       | tee DOCBOOK-BUILD.LOG && \
     $DOCBOOK_SVN/buildtools/build-check DOCBOOK-BUILD.LOG