Use the detected xml2-config script

The AC_CHECK_PROG() macro allows to find the location of a particular
program. In ImageMagick, it is used to find the location of
xml2-config, and fills it into the xml2_config
variable. Unfortunately, the check just below hardcodes `xml2-config
--prefix`, without using the variable, which defeats the whole purpose
of having AC_CHECK_PROG().

So, let's use the ${xml2_config} variable instead. This allows to fix
cross-compilation issues when a non-standard xml2-config location
needs to be specified.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
---
 configure.ac |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: imagemagick-6.6.4-8/configure.ac
===================================================================
--- imagemagick-6.6.4-8.orig/configure.ac
+++ imagemagick-6.6.4-8/configure.ac
@@ -2549,7 +2549,7 @@
         # Debian installs libxml headers under /usr/include/libxml2/libxml with
         # the shared library installed under /usr/lib, whereas the package
         # installs itself under $prefix/libxml and $prefix/lib.
-        xml2_prefix=`xml2-config --prefix`
+        xml2_prefix=`${xml2_config} --prefix`
         if test -d "${xml2_prefix}/include/libxml2"; then
             CPPFLAGS="$CPPFLAGS -I${xml2_prefix}/include/libxml2"
         fi