When building with -flto, we need to use linker plugins.
In case of binutils' nm, it means to prefer gcc-nm if
available.
Like for ranlib and ar, prefer gcc-nm.
- replace AC_PATH_TOOL() by AC_CHECK_TOOLS(). That is consistent
with what we do for ar,ranlib and suggested on bgo#783311.
- instead of using the variable $BINUTILS_NM, replace it by
$NM, which is more common according to bgo#783311.
- Keep recognizing $BINUTILS_NM environment, which was introduced
by commit 8bc88bcc7c. This is purely to keep previous build
scripts working. Originally I named it "$BINUTILS_NM" because
using $NM in NetworkManager seemed confusing. But well...
https://bugs.gentoo.org/show_bug.cgi?id=620052https://bugzilla.gnome.org/show_bug.cgi?id=782525https://bugzilla.gnome.org/show_bug.cgi?id=783311
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