Fix Mac build.

The new line in the set(CMAKE_CXX_FLAGS...) line was getting insert
verbatim into the Makefile. Makefiles don't like new lines in their
assignment operators.

The newline escape fix works for CMake 3.0 and above, but since
we support 2.x, we need to use the legacy solution, which is to
split into two separate statements.

Tested: cmake -G"Unix Makefiles" works now on Mac.
Change-Id: I6f4655981b85087c4760c3d26ed0c97c4469ba93

CMakeLists.txt

@@ -96,9 +96,8 @@ set(FlatBuffers_GRPCTest_SRCS
 # source_group(Tests FILES ${FlatBuffers_Tests_SRCS})
 
 if(APPLE)
-  set(CMAKE_CXX_FLAGS
-    "${CMAKE_CXX_FLAGS} -std=c++11 -stdlib=libc++ -Wall -pedantic -Werror \
-                        -Wextra")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -stdlib=libc++")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -pedantic -Werror -Wextra")
 elseif(CMAKE_COMPILER_IS_GNUCXX)
   if(CYGWIN)
     set(CMAKE_CXX_FLAGS