Adapt sources to configurable config.h name

library/pkparse.c

diff --git a/library/pkparse.c b/library/pkparse.c
index d0ae5d5..ae8bddf 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -23,7 +23,11 @@
  *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#if !defined(POLARSSL_CONFIG_FILE)
 #include "polarssl/config.h"
+#else
+#include POLARSSL_CONFIG_FILE
+#endif
 
 #if defined(POLARSSL_PK_PARSE_C)