When using the (begin, end) constructor for string, the types of begin
and end must match, including cv qualifiers. Depending on the library
headers used, strstr(const char *, const char *) can potentially return
char * or const char *, the former being a call to the C function strstr
and the latter being a call to C++ std::strstr.
This commit fixes the clang portion of issue #21, by explicitly casting
the return value of strstr to match the 1st param of the string ctor so
that the ctor can be instantiated regardless of the header used.
Signed-off-by: Jim Ye <gjimye@gmail.com>