libgmime-2.6-dev entered debian unstable today.  If 2.6 is available,
notmuch should build against 2.6 instead of 2.4, as 2.6 is the current
upstream stable version of libgmime.
---
 debian/control |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/debian/control b/debian/control
index f6f415e..b60790e 100644
--- a/debian/control
+++ b/debian/control
@@ -10,7 +10,7 @@ Build-Depends:
  debhelper (>= 7.0.50~),
  pkg-config,
  libxapian-dev,
- libgmime-2.4-dev,
+ libgmime-2.6-dev | libgmime-2.4-dev,
  libtalloc-dev,
  libz-dev,
  python-all (>= 2.6.6-3~),
-- 
1.7.8.3