Merge bitcoin-core#620: Install headers automatically

16e8615 Install headers automatically (Víctor Mayoral Vilches)

Pull request description:

  This fix install all the headers under include/ into
  /usr/local/include. The fix solves problems that arise
  when building libraries that depend on secp256k1 such
  as libbitcoin-system which require all the headers

Tree-SHA512: 8a5dc664b278e47340bf7478ad278306c44c4c8ad17a023b198c6a927c67c7a7a50100357388342129078afdf7606d2ed06579ce1fc14195fa974510b933021b

Makefile.am

@@ -8,6 +8,8 @@ else
 JNI_LIB =
 endif
 include_HEADERS = include/secp256k1.h
+include_HEADERS += include/secp256k1_ecdh.h
+include_HEADERS += include/secp256k1_recovery.h
 noinst_HEADERS =
 noinst_HEADERS += src/scalar.h
 noinst_HEADERS += src/scalar_4x64.h