Add --with-dri-searchpath argument

* Add an extra argument to configure which allows for specifying
  different DRI driver search paths to libGL (FDO #24766)

Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>
diff --git a/configure.ac b/configure.ac
index 44fb779..5d11cf8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -649,6 +649,13 @@
     [DRI_DRIVER_INSTALL_DIR="$withval"],
     [DRI_DRIVER_INSTALL_DIR='${libdir}/dri'])
 AC_SUBST([DRI_DRIVER_INSTALL_DIR])
+dnl Extra search path for DRI drivers
+AC_ARG_WITH([dri-searchpath],
+    [AS_HELP_STRING([--with-dri-searchpath=DIRS...],
+        [semicolon delimited DRI driver search directories @<:@${libdir}/dri@:>@])],
+    [DRI_DRIVER_SEARCH_DIR="$withval"],
+    [DRI_DRIVER_SEARCH_DIR='${DRI_DRIVER_INSTALL_DIR}'])
+AC_SUBST([DRI_DRIVER_SEARCH_DIR])
 dnl Direct rendering or just indirect rendering
 AC_ARG_ENABLE([driglx-direct],
     [AS_HELP_STRING([--disable-driglx-direct],