tzdata: changed to explicitely inject -std=gnu99 through CFLAGS

Some code seems to depend on gnu extensions in the libc which get found
magically when building a glibc based system.

base/tzdata/tzdata.conf

@@ -2,7 +2,7 @@
 # This copyright note is auto-generated by ./scripts/Create-CopyPatch.
 #
 # Filename: package/.../tzdata/tzdata.conf
-# Copyright (C) 2008 The OpenSDE Project
+# Copyright (C) 2008 - 2013 The OpenSDE Project
 #
 # More information can be found in the files COPYING and README.
 #
@@ -29,7 +29,7 @@ tzdata_extract() {
 }
 
 # they add some options when setting CC in the Makefile, we have to override $(cc)
-makeopt="cc=$CC AWK=$AWK RANLIB=$RANLIB"
+makeopt="cc=$CC CFLAGS="-std=gnu99" AWK=$AWK RANLIB=$RANLIB"
 # use `zic` from the PATH, instead of the just compiled one
 var_append makeopt ' ' 'zic=zic'