Merge #1094: doc: Clarify configure flags for optional modules

55512d3 doc: clean up module help text in configure.ac (Elliott Jin)
d9d94a9 doc: mention optional modules in README (Elliott Jin)

Pull request description:

ACKs for top commit:
  real-or-random:
    utACK 55512d3
  jonasnick:
    ACK 55512d3

Tree-SHA512: ae4ec355730983117c5e9a8a8abd17aaf42afe6f8f8f7474a551df6269a62094883e0827d2f3642e3ed6eb26cf71982c20f7ac27498cb4bd7e4aea57ec308d6a

README.md

@@ -68,12 +68,15 @@ libsecp256k1 is built using autotools:
     $ make check  # run the test suite
     $ sudo make install  # optional
 
+To compile optional modules (such as Schnorr signatures), you need to run `./configure` with additional flags (such as `--enable-module-schnorrsig`). Run `./configure --help` to see the full list of available flags.
+
 Usage examples
 -----------
   Usage examples can be found in the [examples](examples) directory. To compile them you need to configure with `--enable-examples`.
   * [ECDSA example](examples/ecdsa.c)
-  * [Schnorr Signatures example](examples/schnorr.c)
-  * [Deriving a shared secret(ECDH) example](examples/ecdh.c)
+  * [Schnorr signatures example](examples/schnorr.c)
+  * [Deriving a shared secret (ECDH) example](examples/ecdh.c)
+  To compile the Schnorr signature and ECDH examples, you also need to configure with `--enable-module-schnorrsig` and `--enable-module-ecdh`.
 
 Test coverage
 -----------

configure.ac

@@ -141,19 +141,19 @@ AC_ARG_ENABLE(examples,
     [SECP_SET_DEFAULT([enable_examples], [no], [yes])])
 
 AC_ARG_ENABLE(module_ecdh,
-    AS_HELP_STRING([--enable-module-ecdh],[enable ECDH shared secret computation]), [],
+    AS_HELP_STRING([--enable-module-ecdh],[enable ECDH module [default=no]]), [],
     [SECP_SET_DEFAULT([enable_module_ecdh], [no], [yes])])
 
 AC_ARG_ENABLE(module_recovery,
     AS_HELP_STRING([--enable-module-recovery],[enable ECDSA pubkey recovery module [default=no]]), [],
     [SECP_SET_DEFAULT([enable_module_recovery], [no], [yes])])
 
 AC_ARG_ENABLE(module_extrakeys,
-    AS_HELP_STRING([--enable-module-extrakeys],[enable extrakeys module]), [],
+    AS_HELP_STRING([--enable-module-extrakeys],[enable extrakeys module [default=no]]), [],
     [SECP_SET_DEFAULT([enable_module_extrakeys], [no], [yes])])
 
 AC_ARG_ENABLE(module_schnorrsig,
-    AS_HELP_STRING([--enable-module-schnorrsig],[enable schnorrsig module]), [],
+    AS_HELP_STRING([--enable-module-schnorrsig],[enable schnorrsig module [default=no]]), [],
     [SECP_SET_DEFAULT([enable_module_schnorrsig], [no], [yes])])
 
 AC_ARG_ENABLE(external_default_callbacks,