Don't use -Wdouble-promotion gcc option

The warning flag isn't recognized for older GCC versions (blackfin),
so just disable it.

Signed-off-by: Gustavo Zacarias <gustavo.zacarias@free-electrons.com>
[Gustavo: update for 1.3.5]

diff -Nura graphite2-1.3.5.orig/src/CMakeLists.txt graphite2-1.3.5/src/CMakeLists.txt
--- graphite2-1.3.5.orig/src/CMakeLists.txt	2016-02-15 11:46:45.941906112 -0300
+++ graphite2-1.3.5/src/CMakeLists.txt	2016-02-15 11:46:54.237194222 -0300
@@ -111,9 +111,6 @@
         COMPILE_FLAGS   "-Wall -Wextra -Wno-unknown-pragmas -Wendif-labels -Wshadow -Wctor-dtor-privacy -Wnon-virtual-dtor -fno-rtti -fno-exceptions -fvisibility=hidden -fvisibility-inlines-hidden -fno-stack-protector"
         LINK_FLAGS      "-nodefaultlibs ${GRAPHITE_LINK_FLAGS}" 
         LINKER_LANGUAGE C)
-    if (CMAKE_COMPILER_IS_GNUCXX)
-        add_definitions(-Wdouble-promotion)
-    endif (CMAKE_COMPILER_IS_GNUCXX)
     message(STATUS "Compiler ID is: ${CMAKE_CXX_COMPILER_ID}")
     if (${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang")
         add_definitions(-Wimplicit-fallthrough)