Updated README with clang instructions

Fix compilation with clang++

With 'all' and 'test' targets. Quick and dirty but compiles correctly.

The 'inline' declaration was preventing it from being exported.

Several classes and functions were not const-correct. Did the minimal changes
to fix compilation, although there are other const fixes that can be done.

- replaced for_each with range-based for loop
- added missing std:: scope
- for-loop temp now declared outside the loop to be in scope
- removed casts in calls to trim()