configure: export pkg-config includedir variable

Some packages may not define custom cflags, in which case a simple
"pkg-config --cflags" call will return an empty string.
This change will be useful to get a valid include path that can be
used in library checks.

Reviewed-by: Haihao Xiang <haihao.xiang@intel.com>
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -1446,9 +1446,11 @@ test_pkg_config(){
     test_cmd $pkg_config --exists --print-errors $pkg_version || return
     pkg_cflags=$($pkg_config --cflags $pkg_config_flags $pkg)
     pkg_libs=$($pkg_config --libs $pkg_config_flags $pkg)
+    pkg_incdir=$($pkg_config --variable=includedir $pkg_config_flags $pkg)
     check_func_headers "$headers" "$funcs" $pkg_cflags $pkg_libs "$@" &&
         enable $name &&
         set_sanitized "${name}_cflags"    $pkg_cflags &&
+        set_sanitized "${name}_incdir"    $pkg_incdir &&
         set_sanitized "${name}_extralibs" $pkg_libs
 }
 
@@ -1466,7 +1468,8 @@ test_pkg_config_cpp(){
     pkg_incdir=$($pkg_config --variable=includedir $pkg_config_flags $pkg)
     test_cpp_condition "$pkg_incdir/$headers" "$cond" "$@" &&
         enable $name &&
-        set_sanitized "${name}_cflags" $pkg_cflags
+        set_sanitized "${name}_cflags" $pkg_cflags &&
+        set_sanitized "${name}_incdir" $pkg_incdir
 }
 
 check_pkg_config(){