Upgrade to expat 2.2.6.

From `Changes`:

  Release 2.2.6 Sun August 12 2018
        Bug fixes:
       #170 #206  Avoid doing arithmetic with NULL pointers in XML_GetBuffer
       #204 #205  Fix 2.2.5 regression with suspend-resume while parsing
                    a document like '<root/>'

        Other changes:
       #165 #168  Autotools: Fix docbook-related configure syntax error
            #166  Autotools: Avoid grep option `-q` for Solaris
            #167  Autotools: Support
                    ./configure DOCBOOK_TO_MAN="xmlto man --skip-validation"
       #159 #167  Autotools: Support DOCBOOK_TO_MAN command which produces
                    xmlwf.1 rather than XMLWF.1; also covers case insensitive
                    file systems
            #181  Autotools: Drop -rpath option passed to libtool
            #188  Autotools: Detect and deny SGML docbook2man as ours is XML
            #188  Autotools/CMake: Support command db2x_docbook2man as well
            #174  CMake: Introduce option WARNINGS_AS_ERRORS, defaults to OFF
       #184 #185  CMake: Introduce option MSVC_USE_STATIC_CRT, defaults to OFF
       #207 #208  CMake: Introduce option XML_UNICODE and XML_UNICODE_WCHAR_T,
                    both defaulting to OFF
            #175  CMake: Prefer check_symbol_exists over check_function_exists
            #176  CMake: Create the same pkg-config file as with GNU Autotools
       #178 #179  CMake: Use GNUInstallDirs module to set proper defaults for
                    install directories
            #208  CMake: Utilize expat_config.h.cmake for XML_DEV_URANDOM
            #180  Windows: Fix compilation of test suite for Visual Studio 2008
  #131 #173 #202  Address compiler warnings
  #187 #190 #200  Fix miscellaneous typos
                  Version info bumped from 7:7:6 to 7:8:6

        Special thanks to:
            Anton Maklakov
            Benjamin Peterson
            Brad King
            Franek Korta
            Frank Rast
            Joe Orton
            luzpaz
            Pedro Vicente
            Rainer Jung
            Rhodri James
            Rolf Ade
            Rolf Eike Beer
            Thomas Beutlich
            Tomasz Kłoczko

Bug: N/A
Test: cts-tradefed run cts -m CtsLibcoreTestCases -t libcore.xml.ExpatSaxParserTest
Change-Id: I39ea70944ada1553cbc721ff9fd710c1937c7244
diff --git a/configure.ac b/configure.ac
index c24f518..a68d9a4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -47,7 +47,7 @@
 dnl
 
 LIBCURRENT=7   # sync
-LIBREVISION=7  # with
+LIBREVISION=8  # with
 LIBAGE=6       # CMakeLists.txt!
 
 CPPFLAGS="${CPPFLAGS} -DHAVE_EXPAT_CONFIG_H"
@@ -106,8 +106,8 @@
 AS_HELP_STRING([--without-xmlwf], [do not build xmlwf])], [], [with_xmlwf=yes])
 AM_CONDITIONAL([WITH_XMLWF], [test x${with_xmlwf} = xyes])
 
-AM_CONDITIONAL([MINGW], [echo -- "${host}" | fgrep -q mingw])
-AM_CONDITIONAL([UNICODE], [echo -- "${CPPFLAGS}${CFLAGS}" | fgrep -q XML_UNICODE])
+AM_CONDITIONAL([MINGW], [echo -- "${host}" | ${FGREP} mingw >/dev/null])
+AM_CONDITIONAL([UNICODE], [echo -- "${CPPFLAGS}${CFLAGS}" | ${FGREP} XML_UNICODE >/dev/null])
 
 
 AC_ARG_WITH([libbsd], [
@@ -237,11 +237,19 @@
 
 AC_ARG_VAR([DOCBOOK_TO_MAN], [docbook2x-man command])
 AS_IF([test "x$with_docbook" != xno],
-  [AC_CHECK_PROGS([DOCBOOK_TO_MAN], [docbook2x-man docbook2man docbook-to-man],
-  [if test "x$with_docbook" != xcheck; then 
-    AC_MSG_ERROR([Required program 'docbook2x-man' not found.])])])
+  [AC_CHECK_PROGS([DOCBOOK_TO_MAN], [docbook2x-man db2x_docbook2man docbook2man docbook-to-man])])
+AS_IF([test "x${DOCBOOK_TO_MAN}" = x -a "x$with_docbook" = xyes],
+  [AC_MSG_ERROR([Required program 'docbook2x-man' not found.])])
+AS_IF([test "x${DOCBOOK_TO_MAN}" != x -a "x$with_docbook" != xno],
+  [AS_IF([${DOCBOOK_TO_MAN} --help | grep -i -q -F sgmlbase],
+    [AC_MSG_ERROR([Your local ${DOCBOOK_TO_MAN} was found to work with SGML rather
+  than XML. Please install docbook2X and use variable DOCBOOK_TO_MAN to point
+  configure to command docbook2x-man of docbook2X.
+  Or use DOCBOOK_TO_MAN="xmlto man --skip-validation" if you have xmlto around.
+  You can also configure using --without-docbook if you can do without a man
+  page for xmlwf.])])])
 
-AM_CONDITIONAL(WITH_DOCBOOK, [test x${DOCBOOK_TO_MAN} != x])
+AM_CONDITIONAL(WITH_DOCBOOK, [test "x${DOCBOOK_TO_MAN}" != x])
 
 AC_CONFIG_FILES([Makefile expat.pc])
 AC_CONFIG_FILES([