* configure.in (AC_CHECK_FUNCS): Detect library functions

* configure.in (AC_CHECK_FUNCS): Detect library functions
strcasecmp and strncasecmp.
* lib-src/etags.c (etags_strcasecmp, etags_strncasecmp): Define to
library functions strcasecmp and strncasecmp if available.
* lwlib/lwlib.c (my_strcasecmp): Rename to lwlib_strcasecmp, which
may be defined to library function strcasecmp if available.
* src/dispextern.c (xstrcasecmp): Define to library function
strcasecmp if available.
* src/xfaces.c: Do not use xstrcasecmp if strcasecmp is available.