configure.ac: Add --with-crypto= instead of many different --with-$backend

The new --with-crypto option replaces the previous backend-specific
--with-{openssl,libgcrypt,mbedtls,wincng} options and fixes some issues.

* libgcrypt or mbedtls would previously be used whenever found, even
  if configure was passed --without-libgcrypt or --without-mbedtls.

* If --with-$backend was specified then configure would not fail even
  if that library could not be found, and would instead use whichever
  crypto library was found first.

The new option defaults to `auto`, which makes configure check for all
supported crypto libraries in turn, choosing the first one found, or
exiting with an error if none can be found.

docs/HACKING.CRYPTO

@@ -13,6 +13,33 @@ Procedures listed as "void" may indeed have a result type: the void indication
 indicates the libssh2 core modules never use the function result.
 
 
+0) Build system.
+
+Add a new crypto backend to the autotools build system (./configure) as such:
+
+* Add one new line to configure.ac:
+
+m4_set_add([crypto_backends], [newname])
+
+This automatically creates a new --with-crypto=newname option which users can
+specify when invoking configure at compile-time to select the new backend.
+
+* Add a new m4_case stanza to acinclude.m4 within LIBSSH2_CRYPTO_CHECK,
+  with checks for library availability. A successful check should set
+  library linking variables. The LIBSSH2_LIB_HAVE_LINKFLAGS macro creates
+  such a variable automatically if the checked library can be found.
+
+* Add a Makefile.newname.inc in the top-level directory which sets
+  CRYPTO_CSOURCES and CRYPTO_HHEADERS to the new backend source files,
+  and CRYPTO_LTLIBS to the libtool linking parameters for the library, set
+  e.g. by a LIBSSH2_LIB_HAVE_LINKFLAGS call in LIBSSH2_CRYPTO_CHECK.
+
+* Add a new block to src/Makefile.am:
+  if NEWNAME
+  include ../Makefile.newname.inc
+  endif
+
+
 1) Crypto library initialization/termination.
 
 void libssh2_crypto_init(void);