Add const prefix to char16_t*

(AdditiveGlyph): Add const prefix
(convertAlphabetic): Add const prefix
Add signal.h directive

This fixes compile error on darwin environment.
Change isnanf to isnan

(Block::getBaseline, Block::layOut, CellBox::getBaseline, CellBox::adjustWidth, TableWrapperBox::layOut): Change isnanf to isnan
Add AC_CHECK_LIB for boost libraries

configure.ac (AC_CHECK_LIB): Add boost libraries check
Makefile.am: Remove explicit boost libraries link
Convert stream to boolean explicitly

Because of "explicit" of explicit operator bool, clang++ rejects implicit boolean conversion.

(U16TrivialInputStream::operator bool): Add explicit boolean conversion
Support clang++ and libc++ on darwin

Makefile.am: Add STD_CXX11_LIBS and STD_CXX11_CPPFLAGS
configure.ac: Add AC_CANONICAL_HOST, and clang++ and libc++ check
configure.ac: Add char16_t ctype / numpunct stubs