Merge #940: contrib: Explain explicit header guards

22a9ea1 contrib: Explain explicit header guards (Tim Ruffing)

Pull request description:

  They were added in #925 and deserve a comment.

ACKs for top commit:
  gmaxwell:
    ACK 22a9ea1
  sipa:
    ACK 22a9ea1

Tree-SHA512: 832e28d71857d52912dae7e6c0e08a3183bb788996bb2470616c6fbbac6ba601cc74bb51a4c908aec7df9ae4f4cbf2cbb1b451cefde1b5a7359dc93299840278

Author: sipa

contrib/lax_der_parsing.h

@@ -51,6 +51,10 @@
 #ifndef SECP256K1_CONTRIB_LAX_DER_PARSING_H
 #define SECP256K1_CONTRIB_LAX_DER_PARSING_H
 
+/* #include secp256k1.h only when it hasn't been included yet.
+   This enables this file to be #included directly in other project
+   files (such as tests.c) without the need to set an explicit -I flag,
+   which would be necessary to locate secp256k1.h. */
 #ifndef SECP256K1_H
 #include <secp256k1.h>
 #endif

contrib/lax_der_privatekey_parsing.h

@@ -28,6 +28,10 @@
 #ifndef SECP256K1_CONTRIB_BER_PRIVATEKEY_H
 #define SECP256K1_CONTRIB_BER_PRIVATEKEY_H
 
+/* #include secp256k1.h only when it hasn't been included yet.
+   This enables this file to be #included directly in other project
+   files (such as tests.c) without the need to set an explicit -I flag,
+   which would be necessary to locate secp256k1.h. */
 #ifndef SECP256K1_H
 #include <secp256k1.h>
 #endif