bigfile/ram_shmfs: Try to support compiling on older systems

For example on Debian 7 Wheezy the kernel (linux 3.2) supports holepunching,
because holepunching was added in linux 2.6.38:

    http://git.kernel.org/linus/79124f18b335172e1916075c633745e12dae1dac

but glibc does not provide fallocate mode constants, and compilation fails:

    gcc -pthread -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -D_GNU_SOURCE -I./include -I./3rdparty/ccan -I./3rdparty/include -I/opt/slapgrid/f2a5f59d0d2b521681b9333ee76a2859/parts/python2.7/include/python2.7 -c bigfile/ram_shmfs.c -o build/temp.linux-x86_64-2.7/bigfile/ram_shmfs.o -std=gnu99 -fplan9-extensions -fvisibility=hidden -Wno-declaration-after-statement -Wno-error=declaration-after-statement
    bigfile/ram_shmfs.c: In function ‘shmfs_drop_memory’:
    bigfile/ram_shmfs.c:135:31: error: ‘FALLOC_FL_PUNCH_HOLE’ undeclared (first use in this function)
    bigfile/ram_shmfs.c:135:31: note: each undeclared identifier is reported only once for each function it appears in
    bigfile/ram_shmfs.c:135:54: error: ‘FALLOC_FL_KEEP_SIZE’ undeclared (first use in this function)
    error: command 'gcc' failed with exit status 1

Try to fix it via fallbacking to provide the needed defines on older systems ourselves.

Cc: Klaus Wölfel <klaus@nexedi.com>
Cc: Ivan Tyagov <ivan@tyagov.com>

bigfile/ram_shmfs.c

@@ -26,6 +26,17 @@
 #include <wendelin/bug.h>
 
 #include <fcntl.h>
+/* FIXME glibc in Debian before Jessie does not define FALLOC_FL_KEEP_SIZE and
+ * FALLOC_FL_PUNCH_HOLE, even when kernel supports it
+ * http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/falloc.h
+ */
+#ifndef FALLOC_FL_KEEP_SIZE
+# define FALLOC_FL_KEEP_SIZE    0x01
+#endif
+#ifndef FALLOC_FL_PUNCH_HOLE
+# define FALLOC_FL_PUNCH_HOLE   0x02
+#endif
+
 #include <unistd.h>
 #include <sys/vfs.h>
 #include <sys/mman.h>