FindLibXml2: Switch to standard module variable names

* LIBXML2_INCLUDE_DIR talks about libxml2 itself;
* LIBXML2_INCLUDE_DIRS lists also include directories coming from
  libxml2's dependencies and is what libxml2 users should use;
* LIBXML2_LIBRARY talks about libxml2 itself;
* LIBXML2_LIBRARIES is what libxml2 users should link to.

This commit doesn't break existing users of FindLibXml2.cmake,
but provides the way for correct behaviour in future. In particular,
on OpenBSD LIBXML2_INCLUDE_DIR resolves to /usr/local/include/libxml2,
while libiconv (an libxml2 dependency) puts iconv.h
in /usr/local/include, which is not picked up automatically.

Modules/FindLibXml2.cmake

@@ -5,18 +5,37 @@
 # FindLibXml2
 # -----------
 #
-# Try to find the LibXml2 xml processing library
+# Find the XML processing library (libxml2).
 #
-# Once done this will define
+# Result variables
+# ^^^^^^^^^^^^^^^^
 #
-# ::
+# This module will set the following variables in your project:
 #
-#   LIBXML2_FOUND - System has LibXml2
-#   LIBXML2_INCLUDE_DIR - The LibXml2 include directory
-#   LIBXML2_LIBRARIES - The libraries needed to use LibXml2
-#   LIBXML2_DEFINITIONS - Compiler switches required for using LibXml2
-#   LIBXML2_XMLLINT_EXECUTABLE - The XML checking tool xmllint coming with LibXml2
-#   LIBXML2_VERSION_STRING - the version of LibXml2 found (since CMake 2.8.8)
+# ``LIBXML2_FOUND``
+#   true if libxml2 headers and libraries were found
+# ``LIBXML2_INCLUDE_DIR``
+#   the directory containing LibXml2 headers
+# ``LIBXML2_INCLUDE_DIRS``
+#   list of the include directories needed to use LibXml2
+# ``LIBXML2_LIBRARIES``
+#   LibXml2 libraries to be linked
+# ``LIBXML2_DEFINITIONS``
+#   the compiler switches required for using LibXml2
+# ``LIBXML2_XMLLINT_EXECUTABLE``
+#   path to the XML checking tool xmllint coming with LibXml2
+# ``LIBXML2_VERSION_STRING``
+#   the version of LibXml2 found (since CMake 2.8.8)
+#
+# Cache variables
+# ^^^^^^^^^^^^^^^
+#
+# The following cache variables may also be set:
+#
+# ``LIBXML2_INCLUDE_DIR``
+#   the directory containing LibXml2 headers
+# ``LIBXML2_LIBRARY``
+#   path to the LibXml2 library
 
 # use pkg-config to get the directories and then use these values
 # in the find_path() and find_library() calls
@@ -31,7 +50,7 @@ find_path(LIBXML2_INCLUDE_DIR NAMES libxml/xpath.h
    PATH_SUFFIXES libxml2
    )
 
-find_library(LIBXML2_LIBRARIES NAMES xml2 libxml2
+find_library(LIBXML2_LIBRARY NAMES xml2 libxml2
    HINTS
    ${PC_LIBXML_LIBDIR}
    ${PC_LIBXML_LIBRARY_DIRS}
@@ -52,9 +71,12 @@ elseif(LIBXML2_INCLUDE_DIR AND EXISTS "${LIBXML2_INCLUDE_DIR}/libxml/xmlversion.
     unset(libxml2_version_str)
 endif()
 
+set(LIBXML2_INCLUDE_DIRS ${LIBXML2_INCLUDE_DIR} ${PC_LIBXML_INCLUDE_DIRS})
+set(LIBXML2_LIBRARIES ${LIBXML2_LIBRARY})
+
 include(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
 FIND_PACKAGE_HANDLE_STANDARD_ARGS(LibXml2
-                                  REQUIRED_VARS LIBXML2_LIBRARIES LIBXML2_INCLUDE_DIR
+                                  REQUIRED_VARS LIBXML2_LIBRARY LIBXML2_INCLUDE_DIR
                                   VERSION_VAR LIBXML2_VERSION_STRING)
 
-mark_as_advanced(LIBXML2_INCLUDE_DIR LIBXML2_LIBRARIES LIBXML2_XMLLINT_EXECUTABLE)
+mark_as_advanced(LIBXML2_INCLUDE_DIR LIBXML2_LIBRARY LIBXML2_XMLLINT_EXECUTABLE)