Unbundling tinyxml2 from gromacs and using our own, which is newer, broke gromacs
build.

This patch fixes this issue:

- Error handling API changed, fix the testutils/refdata_xml.cpp code by using the
  new API: document.ErrorStr() & tinyxml2::XML_SUCCESS.

Those fixes will be submitted for inclusion to upstream, but may not be suitable
there as long as they still keep the old version bundled.

First hunk has already been requested for merging. Third is in discussion. Second
will only be sent if third is OK'ed.

diff --git a/CMakeLists.txt b/CMakeLists.txt
index cd748c9..1e90c95 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -539,9 +539,6 @@ if(GMX_EXTERNAL_TINYXML2)
     if(NOT HAVE_TINYXML2)
         message(FATAL_ERROR "External TinyXML-2 could not be found, please adjust your search paths")
     endif()
-    if (TinyXML2_FOUND AND TinyXML2_VERSION VERSION_GREATER "6")
-        message(FATAL_ERROR "External TinyXML-2 is later than the highest supported version 6. Please adjust your search paths to include a supported version")
-    endif()
 endif()
 
 option(GMX_EXTRAE "Add support for tracing using EXTRAE" OFF)
diff --git a/src/testutils/refdata_xml.cpp b/src/testutils/refdata_xml.cpp
index 0eb2209..57cebff 100644
--- a/src/testutils/refdata_xml.cpp
+++ b/src/testutils/refdata_xml.cpp
@@ -206,21 +206,12 @@ ReferenceDataEntry::EntryPointer readReferenceDataFile(const std::string& path)
     document.LoadFile(path.c_str());
     if (document.Error())
     {
-        const char* errorStr1 = document.GetErrorStr1();
-        const char* errorStr2 = document.GetErrorStr2();
+        const char* errorStr = document.ErrorStr();
         std::string errorString("Error was ");
-        if (errorStr1)
-        {
-            errorString += errorStr1;
-        }
-        if (errorStr2)
-        {
-            errorString += errorStr2;
-        }
-        if (!errorStr1 && !errorStr2)
-        {
+        if (errorStr)
+            errorString += errorStr;
+        else
             errorString += "not specified.";
-        }
         GMX_THROW(TestException("Reference data not parsed successfully: " + path + "\n."
                                 + errorString + "\n"));
     }
@@ -371,7 +362,7 @@ void writeReferenceDataFile(const std::string& path, const ReferenceDataEntry& r
     XMLElementPtr rootElement = createRootElement(&document);
     createChildElements(rootElement, rootEntry);
 
-    if (document.SaveFile(path.c_str()) != tinyxml2::XML_NO_ERROR)
+    if (document.SaveFile(path.c_str()) != tinyxml2::XML_SUCCESS)
     {
         GMX_THROW(TestException("Reference data saving failed in " + path));
     }
r>
<tr><td><span title='2018-03-23 04:41:51 -0400'>2018-03-23</span></td><td><a href='/guix/commit/gnu/packages/gawk.scm?id=dcd5ea699fe4f7e8c1f53933e4d0e027c1a0a512'>gnu: gawk: Return #t from all phases.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/gawk.scm (gawk)[arguments]: Return #t from
the 'set-shell-file-name' phase.
</span></span></td><td>Mark H Weaver</td></tr>
<tr><td><span title='2018-03-14 19:12:06 +0200'>2018-03-14</span></td><td><a href='/guix/commit/gnu/packages/gawk.scm?id=7c4a0f975b587d0c2718b80ba5c2fa7b8cc8f2ec'>gnu: gawk: Enable parallel tests.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/gawk.scm (gawk)[arguments]: Enable parallel tests.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2018-03-14 19:12:06 +0200'>2018-03-14</span></td><td><a href='/guix/commit/gnu/packages/gawk.scm?id=3ba42b8566704c588f8d76828f960575c6fc0f53'>gnu: gawk: Update to 4.2.1.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/gawk.scm (gawk): Update to 4.2.1.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-12-10 23:00:42 +0200'>2017-12-10</span></td><td><a href='/guix/commit/gnu/packages/gawk.scm?id=173c6ec8a391dee9d7b38a20370e48a4a05288d0'>Revert "gnu: gawk: Update to 4.2.0."</a><span class='msg-avail'>...<span class='msg-tooltip'>This reverts commit 78cd051eef9e25a6993478619d2bdb2db60565a6.

This commit was added past the merge window and caused conflicts with
gettext's test suite.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-12-10 11:21:43 +0200'>2017-12-10</span></td><td><a href='/guix/commit/gnu/packages/gawk.scm?id=78cd051eef9e25a6993478619d2bdb2db60565a6'>gnu: gawk: Update to 4.2.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/gawk.scm (gawk): Update to 4.2.0.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-03-30 22:59:53 +0200'>2017-03-30</span></td><td><a href='/guix/commit/gnu/packages/gawk.scm?id=84157bb8bf2c610584e0836047da4c710f8eaf76'>Merge branch 'master' into core-updates</a><span class='msg-avail'>...<span class='msg-tooltip'>Most conflicts are from 6fd52309b8f52c9bb59fccffac53e029ce94b698.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2017-03-30 01:30:16 +0200'>2017-03-30</span></td><td><a href='/guix/commit/gnu/packages/gawk.scm?id=6fd52309b8f52c9bb59fccffac53e029ce94b698'>gnu: Use HTTPS for almost all gnu.org HOME-PAGEs.</a><span class='msg-avail'>...<span class='msg-tooltip'>All HTTP gnu.org (and supported subdomain) HOME-PAGEs changed to HTTPS.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>