Links to pthread only in LLVM mode

llvm-config utility provides required libraries to link to.
We do not need to specify pthread ourselves.

Issue: #27
Adds const specifier to isSmallInteger and TObject::cast<T>()

Issue: #27
Adds specifying comment to TArray::operator[]

Issue: #27
Fixes s/Lo/Low/ in source files copyleft notice

Issue: #10
Adds copyleft notice to source files

Issue: #10
Merge branches 'fix/10/legal_info' and '27/pre_release_fixes' into develop

* fix/10/legal_info:
  Adds copyleft notice to source files
  Fixes s/Lo/Low/ in source files copyleft notice

* 27/pre_release_fixes:
  Adds specifying comment to TArray::operator[]
  Adds const specifier to isSmallInteger and TObject::cast<T>()
  Links to pthread only in LLVM mode

Issue: #10
Issue: #27