ios: use MIN_IOS_VERSION env variable if possible

This patch adds the ability to use an environment variable as the
MIN_IOS_VERSION.
It still allows to build the daemon in standalone with the variable
set in the main.mak file of the contribs.

Change-Id: I107353e3f1bbc00ccd54c671dc73a3d6b7ad1d6c
Reviewed-by: Alexandre Lision <alexandre.lision@savoirfairelinux.com>

contrib/src/main.mak

@@ -154,7 +154,10 @@ endif
 CCAS=$(CC) -c
 
 ifdef HAVE_IOS
-MIN_IOS_VERSION=8.0
+
+ifndef MIN_IOS_VERSION
+MIN_IOS_VERSION=9.3
+endif
 
 CC=xcrun clang
 CXX=xcrun clang++