Fix warnings reported by -Wpedantic

Previously CI hasn't been running with -Wpedantic (using autoconf), so
these haven't been reported previously.

- replace BUILD_ASSERT with static_assert()
- don't use %m GNU extension for printf
- don't use arithmetic on void*
- remove unused variables

Change-Id: I97d1acc908f06773b8b1ee95bfee80760fdc7a63
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
diff --git a/astlpc.c b/astlpc.c
index 6de3e37..d322b7c 100644
--- a/astlpc.c
+++ b/astlpc.c
@@ -1401,7 +1401,8 @@
 		astlpc_prwarn(astlpc, "LPC mmap failed");
 		rc = -1;
 	} else {
-		astlpc->lpc_map = lpc_map_base + map.size - LPC_WIN_SIZE;
+		astlpc->lpc_map =
+			(uint8_t *)lpc_map_base + map.size - LPC_WIN_SIZE;
 	}
 
 	close(fd);