small: fix small compilation

If SMALL_EMBEDDED is used, "small" will try to compile
with the default flags, but it can't be compiled with std=c89.
Look like a "small" library bug.
So, we will use std=c99 globally as workaround.

See tarantool/small#25

Part of #59

@Totktonada: added the comment.

CMakeLists.txt

@@ -15,7 +15,13 @@ find_package(CyrusSASL)
 # include(cmake/FindTarantool.cmake)
 include_directories(${TARANTOOL_INCLUDE_DIRS})
 
-set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fPIC")
+# We should add -std=c99 before small library inclusion, because
+# the library does not enforce a C dialect, but unable to be built
+# with -std=gnu90 (which is default on GCC 4.8, which is shipped
+# with Ubuntu Trusty).
+#
+# See https://github.com/tarantool/small/issues/25
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fPIC -std=c99")
 
 find_program(GIT git)
 if (EXISTS "${CMAKE_SOURCE_DIR}/.git" AND GIT)