properly set up the sysroot for e.g. check_headers

[mgerhardy]

without this, cmake wasn't able to find any header for me, it got unresolved linker problems with the non-standalone toolchain:

• crtbegin_dynamic.o
• crtend_android.o
• cannot find -lc
• cannot find -ldl

all of the above mentioned issues are fixed by this.

[mgerhardy]

this might also fix it: https://github.com/dreifachstein/android-cmake/commit/785c889dd5871204b7f4dc8c0f52970f2359b809

but this patch should also work find on older cmake versions.

[mgerhardy]

link #70

[mgerhardy]

70 doesn't fix the problem for me alone - both patches in combination worked though.