Handle building with '-fno-common' for gcc 10.
See: https://bugs.gentoo.org/706356


--- a/src/dav_coda.c
+++ b/src/dav_coda.c
@@ -99,3 +99,3 @@ static size_t alignment;
 /* Send debug messages to syslog if != 0. */
-int debug;
+static int debug;
 
--- a/src/dav_fuse.c
+++ b/src/dav_fuse.c
@@ -86,3 +86,3 @@ static uint64_t root;
 /* Send debug messages to syslog if != 0. */
-int debug;
+static int debug;
 
