cmake: require c99 standard

Some files, e.g. encode_accessors.c, use loop initial declarations
that are only allowed in C99 mode. Newer compilers enable that mode
automatically, but olders might fail. CMAKE_C_STANDARD got add in
CMake-3.1, so let's increase the minimal version by 0.1.
Signed-off-by: Christoph Junghans <junghans@votca.org>

CMakeLists.txt

-cmake_minimum_required(VERSION 3.0.0)
+cmake_minimum_required(VERSION 3.1)
+set(CMAKE_C_STANDARD 99)
 include(CheckIncludeFiles)
 include(TestBigEndian)
 include(CheckCSourceCompiles)