fix clang compiling error

This patch fixes Travis error:
```
clang -Imupdf/include -Lbuild/x86_64-unknown-linux-gnu/libs -O2
-ffast-math -pipe -fomit-frame-pointer -D__extern_always_inline=inline
-Qunused-arguments -march=native -fPIC -Iluajit-2.0/src -shared -Wl,-E
-Wl,-rpath,'$ORIGIN' \
                -o build/x86_64-unknown-linux-gnu/libs/libwrap-mupdf.so
                wrap-mupdf.c wrap-mupdf.h
                build/x86_64-unknown-linux-gnu/libs/libmupdf.so
                build/x86_64-unknown-linux-gnu/libs/libk2pdfopt.so.2
clang: error: cannot specify -o when generating multiple output files
make: *** [build/x86_64-unknown-linux-gnu/libs/libwrap-mupdf.so] Error 1
```