Add an override for bison

glibc build requires GNU bison >= 2.7 but Apple ships bison 2.3. If we
provide an override for GNU bison it should be possible to use a version
from homebrew.

Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>

configure.ac

@@ -113,8 +113,8 @@ AC_PROG_RANLIB
 CTNG_PATH_TOOL_REQ([OBJCOPY], [gobjcopy objcopy], [objcopy])
 CTNG_PATH_TOOL_REQ([OBJDUMP], [gobjdump objdump], [objdump])
 CTNG_PATH_TOOL_REQ([READELF], [greadelf readelf], [readelf])
+CTNG_PATH_TOOL_REQ([BISON], [bison], [bison])
 
-CTNG_CHECK_PROGS_REQ([bison], [bison])
 CTNG_CHECK_PROGS_REQ([flex], [flex])
 CTNG_CHECK_PROGS_REQ([makeinfo], [makeinfo])
 CTNG_CHECK_PROGS_REQ([cut], [cut])

paths.sh.in

@@ -1,5 +1,6 @@
 export install="@INSTALL@"
 export bash="@BASH_SHELL@"
+export bison="@BISON@"
 export awk="@AWK@"
 export grep="@GREP@"
 export gmake="@MAKE@"