Autoregenerate libxml2.syms automated checkings

* doc/symbols.xml: the source for all exported symbols
* doc/checkapisym.xsl: used to check libxml2-api.xml against exported
  symbols
* doc/syms.xsl: stylesheet regenerating libxml2.syms from symbols.xml
* doc/Makefile.am libxml2.syms: add the new rules and the generated
  version
diff --git a/doc/checkapisym.xsl b/doc/checkapisym.xsl
new file mode 100644
index 0000000..31a050d
--- /dev/null
+++ b/doc/checkapisym.xsl
@@ -0,0 +1,48 @@
+<?xml version="1.0"?>
+<!-- This stylesheet is used to check that symbols exported
+     from libxml2-api.xml are also present in the symbol file
+     symbols.xml which is used to generate libxml2.syms setting
+     up the allowed access point to the shared libraries -->
+
+
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
+                version="1.0">
+  <xsl:output method="text" encoding="UTF-8"/>
+
+  <xsl:variable name="syms" select="document('symbols.xml')"/>
+
+  <xsl:template match="/">
+    <xsl:message terminate="no">
+      <xsl:text>Looking for functions in symbols.xml</xsl:text>
+    </xsl:message>
+    <xsl:apply-templates select="/api/symbols/function"/>
+    <xsl:message terminate="no">
+      <xsl:text>Found </xsl:text>
+      <xsl:value-of select="count(/api/symbols/function)"/>
+      <xsl:text>functions</xsl:text>
+    </xsl:message>
+    <xsl:message terminate="no">
+      <xsl:text>Looking for variables in symbols.xml</xsl:text>
+    </xsl:message>
+    <xsl:apply-templates select="/api/symbols/variable"/>
+    <xsl:message terminate="no">
+      <xsl:text>Found </xsl:text>
+      <xsl:value-of select="count(/api/symbols/variable)"/>
+      <xsl:text>variables</xsl:text>
+    </xsl:message>
+  </xsl:template>
+
+  <xsl:template match="function|variable">
+    <xsl:variable name="name" select="@name"/>
+    <xsl:variable name="symbol"
+        select="$syms/symbols/release/symbol[. = $name]"/>
+    <xsl:if test="string($symbol) != $name">
+      <xsl:message terminate="yes">
+        <xsl:text>Failed to find export in symbols.xml: </xsl:text>
+        <xsl:value-of select="$name"/>
+      </xsl:message>
+    </xsl:if>
+  </xsl:template>
+
+</xsl:stylesheet>
+