configure: session key handling in gmime maps to built_with("session_key")

This flag should make it easier to write the code for session-key
handling.

Note that this only works for GMime 2.6.21 and later (the session key
interface wasn't available before then).  It should be fine to build
the rest of notmuch if this functionality isn't available.

Note that this also adds the "session_key" built_with() aspect to
libnotmuch.

configure

@@ -486,14 +486,21 @@ if pkg-config --exists "gmime-3.0"; then
     gmime_cflags=$(pkg-config --cflags gmime-3.0)
     gmime_ldflags=$(pkg-config --libs gmime-3.0)
     gmime_major=3
+    have_gmime_session_keys=1
 elif pkg-config --exists "gmime-2.6 >= $GMIME_MINVER"; then
     printf "Yes (2.6).\n"
     have_gmime=1
     gmime_cflags=$(pkg-config --cflags gmime-2.6)
     gmime_ldflags=$(pkg-config --libs gmime-2.6)
     gmime_major=2
+    if pkg-config --exists "gmime-2.6 >= 2.6.21"; then
+        have_gmime_session_keys=1
+    else
+        have_gmime_session_keys=0
+    fi
 else
     have_gmime=0
+    have_gmime_session_keys=0
     printf "No.\n"
     errors=$((errors + 1))
 fi
@@ -1096,6 +1103,9 @@ HAVE_TIMEGM = ${have_timegm}
 # Whether struct dirent has d_type (if not, then notmuch will use stat)
 HAVE_D_TYPE = ${have_d_type}
 
+# Whether the GMime version can handle extraction and reuse of session keys
+HAVE_GMIME_SESSION_KEYS = ${have_gmime_session_keys}
+
 # Whether the Xapian version in use supports compaction
 HAVE_XAPIAN_COMPACT = ${have_xapian_compact}
 
@@ -1181,6 +1191,7 @@ COMMON_CONFIGURE_CFLAGS = \\
 	-DHAVE_D_TYPE=\$(HAVE_D_TYPE)				\\
 	-DSTD_GETPWUID=\$(STD_GETPWUID)				\\
 	-DSTD_ASCTIME=\$(STD_ASCTIME)				\\
+	-DHAVE_GMIME_SESSION_KEYS=\$(HAVE_GMIME_SESSION_KEYS)	\\
 	-DHAVE_XAPIAN_COMPACT=\$(HAVE_XAPIAN_COMPACT)		\\
 	-DSILENCE_XAPIAN_DEPRECATION_WARNINGS			\\
 	-DHAVE_XAPIAN_FIELD_PROCESSOR=\$(HAVE_XAPIAN_FIELD_PROCESSOR) \\
@@ -1209,6 +1220,9 @@ NOTMUCH_HAVE_XAPIAN_FIELD_PROCESSOR=${have_xapian_field_processor}
 # Whether the Xapian version in use supports lock retry
 NOTMUCH_HAVE_XAPIAN_DB_RETRY_LOCK=${have_xapian_db_retry_lock}
 
+# Whether the GMime version can handle extraction and reuse of session keys
+NOTMUCH_HAVE_GMIME_SESSION_KEYS=${have_gmime_session_keys}
+
 # Which backend will Xapian use by default?
 NOTMUCH_DEFAULT_XAPIAN_BACKEND=${default_xapian_backend}
 

lib/built-with.c

@@ -30,6 +30,8 @@ notmuch_built_with (const char *name)
 	return HAVE_XAPIAN_FIELD_PROCESSOR;
     } else if (STRNCMP_LITERAL (name, "retry_lock") == 0) {
 	return HAVE_XAPIAN_DB_RETRY_LOCK;
+    } else if (STRNCMP_LITERAL (name, "session_key") == 0) {
+	return HAVE_GMIME_SESSION_KEYS;
     } else {
 	return false;
     }