Set some compiler flags only for gcc as they are not known to clang

Fixes warnings from clang:

warning: unknown warning option '-Wduplicated-cond'; did you mean '-Wduplicate-enum'? [-Wunknown-warning-option]
warning: unknown warning option '-Wduplicated-branches' [-Wunknown-warning-option]
warning: unknown warning option '-Wlogical-op'; did you mean '-Wlong-long'? [-Wunknown-warning-option]
warning: unknown warning option '-Wrestrict' [-Wunknown-warning-option]
warning: unknown warning option '-Wuseless-cast' [-Wunknown-warning-option]