mesa: make makedepend an hard requirement

Currently makedepend is used by the Mesa Makefile-based build system,
but not required.

Unfortunately, not having it makes dependency resolution non-existent,
which is a source of subtle bugs, and is a rarely tested
configuration, since all Mesa developers likely have it installed.

Furthermore some idioms require dependency resolution to work at all,
such as making headers depend on generated files.

configure.ac

@@ -33,6 +33,10 @@ AC_CHECK_PROGS([MAKE], [gmake make])
 AC_PATH_PROG([MKDEP], [makedepend])
 AC_PATH_PROG([SED], [sed])
 
+if test "x$MKDEP" = "x"; then
+    AC_MSG_ERROR([makedepend is required to build Mesa])
+fi
+
 dnl Our fallback install-sh is a symlink to minstall. Use the existing
 dnl configuration in that case.
 AC_PROG_INSTALL