Assorted fixes and cleanup

The biggest change is actually enabling the USE_XFT code that's been in since the import from XFree86.

• lex.c: increment length counters when adding to buffer
• Replace strcpy() calls with strncpy() or memcpy()
• Add bounds check to getstr()
• More variable scope reductions as recommended by cppcheck
• Initialize more variables in their declarations
• configure: Add --with-xft and --without-xft flags to control USE_XFT
• Mark more char * as const
• Add CWARNFLAGS to AM_CFLAGS
• GetLine: resolve -Wmaybe-uninitialized warning