cmake: fix build when LLVM_INCLUDE_DIRS includes multiple directories (#709)

If LLVM_INCLUDE_DIRS includes multiple directories, separated by
semicolon, the string would be incorrectly propagated all the way down
to the shell, that would interpret such semicolon as a command
separator. E.g. we would have:

 c++ ... -isystem /w/llvm/include;/w/llvm/bld/include ...

Instead, we need to parse the string as a CMake list (that are defined
as strings composed by semicolon-separated tokens) and build a string
in the form:

 c++ ... -isystem /w/llvm/include -isystem /w/llvm/bld/include ...

This bug was introduced in d19e0cb0.
This commit fixes #707.

Signed-off-by: Marco Leogrande <marcol@plumgrid.com>