Trac #29631: new spkg-configure.m4 for linbox.

This is essentially the file posted as an attachment on Trac 29631 by
Thierry Thomas. I've chosen to leave the lower bound at the latest
v1.6.3, since in the intervening months most distributions appear to
have updated.

build/pkgs/linbox/spkg-configure.m4

@@ -0,0 +1,8 @@
+SAGE_SPKG_CONFIGURE([linbox], [
+  SAGE_SPKG_DEPCHECK([fflas-ffpack flint fplll givaro gmp iml m4ri m4rie mpfr ntl], [
+    PKG_CHECK_MODULES([LINBOX],
+                      [linbox >= 1.6.3],
+                      [],
+                      [sage_spkg_install_linbox=yes])
+  ])
+])