Add support for alternative ECJPAKE implementation

This commit allows users to provide alternative implementations of the
ECJPAKE interface through the configuration option MBEDTLS_ECJPAKE_ALT.
When set, the user must add `ecjpake_alt.h` declaring the same
interface as `ecjpake.h`, as well as add some compilation unit which
implements the functionality. This is in line with the preexisting
support for alternative implementations of other modules.
diff --git a/library/version_features.c b/library/version_features.c
index ede2276..72afec2 100644
--- a/library/version_features.c
+++ b/library/version_features.c
@@ -108,6 +108,9 @@
 #if defined(MBEDTLS_DHM_ALT)
     "MBEDTLS_DHM_ALT",
 #endif /* MBEDTLS_DHM_ALT */
+#if defined(MBEDTLS_ECJPAKE_ALT)
+    "MBEDTLS_ECJPAKE_ALT",
+#endif /* MBEDTLS_ECJPAKE_ALT */
 #if defined(MBEDTLS_GCM_ALT)
     "MBEDTLS_GCM_ALT",
 #endif /* MBEDTLS_GCM_ALT */