Task #11558: Removed '-D_GLIBCXX_USE_CXX11_ABI=0' from GNU.cmake.

Since it sets -std=c++98, it won't use the new C++11 ABI anyway. 
More important though, it breaks compilation with GCC-5, because several
header files simply check if _GLIBCXX_USE_CXX11_ABI is defined or not.

CMake/variants/GNU.cmake

@@ -21,11 +21,11 @@ set(GNU_C_FLAGS_DEBUG    "-g")
 set(GNU_C_FLAGS_OPT      "-g -O2")
 set(GNU_C_FLAGS_OPT3     "-g -O3")
 set(GNU_C_FLAGS_OPTARCH  "-g -O3 -march=native")
-set(GNU_CXX_FLAGS        "-std=c++98 -D_GLIBCXX_USE_CXX11_ABI=0 -W -Wall -Woverloaded-virtual -Wno-unknown-pragmas")
-set(GNU_CXX_FLAGS_DEBUG  "-std=c++98 -D_GLIBCXX_USE_CXX11_ABI=0 -g")
-set(GNU_CXX_FLAGS_OPT    "-std=c++98 -D_GLIBCXX_USE_CXX11_ABI=0 -g -O2")
-set(GNU_CXX_FLAGS_OPT3   "-std=c++98 -D_GLIBCXX_USE_CXX11_ABI=0 -g -O3")
-set(GNU_CXX_FLAGS_OPTARCH "-std=c++98 -D_GLIBCXX_USE_CXX11_ABI=0 -g -O3 -march=native")
+set(GNU_CXX_FLAGS        "-std=c++98 -W -Wall -Woverloaded-virtual -Wno-unknown-pragmas")
+set(GNU_CXX_FLAGS_DEBUG  "-std=c++98 -g")
+set(GNU_CXX_FLAGS_OPT    "-std=c++98 -g -O2")
+set(GNU_CXX_FLAGS_OPT3   "-std=c++98 -g -O3")
+set(GNU_CXX_FLAGS_OPTARCH "-std=c++98 -g -O3 -march=native")
 set(GNU_EXE_LINKER_FLAGS)
 set(GNU_EXE_LINKER_FLAGS_DEBUG)
 set(GNU_EXE_LINKER_FLAGS_OPT)