Add flag to forcibly disable fallocate

Summary:
see https://github.com/facebook/rocksdb/issues/977; there are issues
with fallocate() on certain filesystems/kernel versions that can lead it to pre-
allocating blocks but never freeing them, even if they're unused.

Test Plan:
verified build commands omit DROCKSDB_FALLOCATE_PRESENT when this env
variable is set.

without disabling it:

  $ ROCKSDB_NO_FBCODE=1 make -n env_test | grep -q DROCKSDB_FALLOCATE_PRESENT ; echo $?
  0

with disabling it:

  $ ROCKSDB_NO_FBCODE=1 DISABLE_FALLOCATE=1 make -n env_test | grep -q DROCKSDB_FALLOCATE_PRESENT ; echo $?
  1

Reviewers: kradhakrishnan, yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D54069

build_tools/build_detect_platform

@@ -189,6 +189,7 @@ if [ "$CROSS_COMPILE" = "true" -o "$FBCODE_BUILD" = "true" ]; then
     # Also don't need any compilation tests if compiling on fbcode
     true
 else
+    if ! test $ROCKSDB_DISABLE_FALLOCATE; then
         # Test whether fallocate is available
         $CXX $CFLAGS -x c++ - -o /dev/null 2>/dev/null  <<EOF
           #include <fcntl.h>
@@ -201,6 +202,7 @@ EOF
         if [ "$?" = 0 ]; then
             COMMON_FLAGS="$COMMON_FLAGS -DROCKSDB_FALLOCATE_PRESENT"
         fi
+    fi
 
     # Test whether Snappy library is installed
     # http://code.google.com/p/snappy/