build.sh: explicitly disable GDB unit tests

These are disabled by default when `development=false`, which is set
by `binutils/bfd/development.sh`.  However, while `gdb/configure.ac`
includes this file, `gdbsupport/configure.ac` does not, and thus the
two disagree on whether unit tests shall be built.  This causes linker
failures due to missing symbols.

By explicitly disabling the unit tests (or enabling them), both
`configure` scripts agree on the setting and the build succeeds.
Since we don't run the unit tests (and this is not a gdb development
build), we should simply disable them.

Closes https://github.com/MaxKellermann/cegcc-build/issues/6

build.sh

@@ -234,6 +234,7 @@ build_binutils()
     echo ""
     echo ""
     configure_host_module binutils binutils \
+	--disable-unit-tests \
 	--disable-nls		\
 	--disable-werror