gcc-configure-cross: factor out --enable-threads argument into ${GCCTHREADS}

This allows BSPs for architectures with no thread support to set (for
example) "GCCTHREADS=no" without having to override all the other configure
parameters.

(From OE-Core rev: 6bb0d37529a82b953d374f2d76c2412d7cee587b)

Signed-off-by: Phil Blundell <pb@pbcl.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/gcc/gcc-configure-common.inc

@@ -26,12 +26,13 @@ EXTRA_OECONF_INITIAL ?= ""
 EXTRA_OECONF_INTERMEDIATE ?= ""
 
 GCCMULTILIB = "--disable-multilib"
+GCCTHREADS ?= "posix"
 
 EXTRA_OECONF = "${@['--enable-clocale=generic', ''][d.getVar('USE_NLS', True) != 'no']} \
                 --with-gnu-ld \
                 --enable-shared \
                 --enable-languages=${LANGUAGES} \
-                --enable-threads=posix \
+                --enable-threads=${GCCTHREADS} \
                 ${GCCMULTILIB} \
                 --enable-c99 \
                 --enable-long-long \