[LyX/master] Fix --with-extra-inc and --with-extra-prefix.

[Jean-Marc]

commit 8fcbba8596f62fdbb1a87b1ee3ba5af1f1571487
Author: Jean-Marc <lasgout...@lyx.org>
Date:   Tue Jan 27 23:07:54 2015 +0100

    Fix --with-extra-inc and --with-extra-prefix.
    
    From Enrico.

diff --git a/configure.ac b/configure.ac
index 8b48eaf..5f5d562 100644
--- a/configure.ac
+++ b/configure.ac
@@ -92,12 +92,12 @@ test ! x"$lyx_ldflags" = x && LDFLAGS="$lyx_ldflags 
$LDFLAGS"
 ### Add extra directories to check for include files.
 LYX_WITH_DIR([extra-inc],[extra include directory],extra_inc, NONE)
 LYX_LOOP_DIR($lyx_cv_extra_inc,LYX_ADD_INC_DIR(lyx_cppflags,$dir))
-test ! x"$lyx_cppflags" = x && AM_CPPFLAGS="$lyx_cppflags $AM_CPPFLAGS"
+test ! x"$lyx_cppflags" = x && CPPFLAGS="$lyx_cppflags $CPPFLAGS"
 
 ### Add both includes and libraries
 LYX_WITH_DIR([extra-prefix],[extra lib+include directory],extra_prefix, NONE, 
${prefix})
 LYX_LOOP_DIR($lyx_cv_extra_prefix,[
-  LYX_ADD_INC_DIR(AM_CPPFLAGS,$dir/include)
+  LYX_ADD_INC_DIR(CPPFLAGS,$dir/include)
   LYX_ADD_LIB_DIR(LDFLAGS,$dir/lib)
 ])