Added infrastructure for different encode modules.

Made tcfs_errno thread-local and added TLS autoconf check.

configure.ac

@@ -23,6 +23,7 @@ AC_CHECK_HEADERS([fcntl.h stddef.h stdlib.h string.h errno.h dirent.h])
 # Checks for typedefs, structures, and compiler characteristics.
 AC_PROG_CC_C99
 AM_PROG_CC_C_O
+AX_TLS
 
 # Checks for library functions.
 AC_CHECK_FUNCS([memset strerror])