ncurses: simplify patch expr, fix ncurses6 build w/clang

Somewhat amusingly given its name, "clang.patch" applies to both 5 and 6
but is the cause of ncurses6 breakage on 6 but is required on 5...
gcc is happy in all four configurations:

       5  5p 6  6p
gcc    ✓  ✓  ✓  ✓
clang  ✗  ✓  ✓  ✗

Which is why this commit enables the patch for 5 but not 6;
this matches behavior in Gentoo, for example.

For further simplification, we also use gcc-5 patch regardless.