Fix DUK_F_CLANG detection to avoid also DUK_F_GCC
Fix DUK_F_CLANG detection to avoid also DUK_F_GCC

Before this fix, both DUK_F_GCC and DUK_F_CLANG would be defined when
compiling with clang.  This might have had an impact on the #ifdef
ladders in duk_features.h.in so that this change may have some side
effects for clang.

zeropool/duktape,sloth4413/duktape,daimajia/duktape,reqshark/duktape,daimajia/duktape,eddieh/duktape,markand/duktape,kphillisjr/duktape,zeropool/duktape,pombredanne/duktape,harold-b/duktape,thurday/duktape,harold-b/duktape,rwaldron/duktape,jmptrader/duktape,chenyaqiuqiu/duktape,tassmjau/duktape,markand/duktape,rwaldron/duktape,pombredanne/duktape,reqshark/duktape,reqshark/duktape,svaarala/duktape,zeropool/duktape,harold-b/duktape,svaarala/duktape,sloth4413/duktape,rwaldron/duktape,nivertech/duktape,pombredanne/duktape,tassmjau/duktape,jmptrader/duktape,tassmjau/duktape,zeropool/duktape,jmptrader/duktape,markand/duktape,sloth4413/duktape,daimajia/duktape,haosu1987/duktape,thurday/duktape,pombredanne/duktape,haosu1987/duktape,reqshark/duktape,haosu1987/duktape,chenyaqiuqiu/duktape,sloth4413/duktape,daimajia/duktape,thurday/duktape,zeropool/duktape,sloth4413/duktape,tassmjau/duktape,skomski/duktape,skomski/duktape,eddieh/duktape,haosu1987/duktape,markand/duktape,skomski/duktape,pombredanne/duktape,jmptrader/duktape,markand/duktape,eddieh/duktape,kphillisjr/duktape,zeropool/duktape,nivertech/duktape,zeropool/duktape,nivertech/duktape,kphillisjr/duktape,harold-b/duktape,haosu1987/duktape,harold-b/duktape,jmptrader/duktape,reqshark/duktape,eddieh/duktape,svaarala/duktape,thurday/duktape,daimajia/duktape,eddieh/duktape,nivertech/duktape,skomski/duktape,thurday/duktape,nivertech/duktape,nivertech/duktape,pombredanne/duktape,kphillisjr/duktape,tassmjau/duktape,tassmjau/duktape,zeropool/duktape,eddieh/duktape,harold-b/duktape,kphillisjr/duktape,reqshark/duktape,markand/duktape,chenyaqiuqiu/duktape,tassmjau/duktape,thurday/duktape,jmptrader/duktape,daimajia/duktape,zeropool/duktape,markand/duktape,skomski/duktape,jmptrader/duktape,harold-b/duktape,eddieh/duktape,nivertech/duktape,sloth4413/duktape,reqshark/duktape,haosu1987/duktape,skomski/duktape,svaarala/duktape,chenyaqiuqiu/duktape,sloth4413/duktape,haosu1987/duktape,haosu1987/duktape,nivertech/duktape,pombredanne/duktape,reqshark/duktape,rwaldron/duktape,sloth4413/duktape,eddieh/duktape,nivertech/duktape,pombredanne/duktape,sloth4413/duktape,haosu1987/duktape,thurday/duktape,thurday/duktape,zeropool/duktape,tassmjau/duktape,chenyaqiuqiu/duktape,markand/duktape,skomski/duktape,kphillisjr/duktape,kphillisjr/duktape,svaarala/duktape,svaarala/duktape,nivertech/duktape,skomski/duktape,harold-b/duktape,eddieh/duktape,harold-b/duktape,chenyaqiuqiu/duktape,skomski/duktape,svaarala/duktape,rwaldron/duktape,markand/duktape,rwaldron/duktape,kphillisjr/duktape,skomski/duktape,kphillisjr/duktape,thurday/duktape,markand/duktape,tassmjau/duktape,tassmjau/duktape,harold-b/duktape,reqshark/duktape,pombredanne/duktape,pombredanne/duktape,thurday/duktape,eddieh/duktape,haosu1987/duktape,svaarala/duktape,sloth4413/duktape,chenyaqiuqiu/duktape,reqshark/duktape,jmptrader/duktape,svaarala/duktape,chenyaqiuqiu/duktape,kphillisjr/duktape,chenyaqiuqiu/duktape,chenyaqiuqiu/duktape,jmptrader/duktape,jmptrader/duktape