Make the PYTHON_SHEBANG value configurable

and gitignore myrocks_hotbackup (as it's now generated)

Closes #1081

Author: vuvova

.gitignore

@@ -189,6 +189,7 @@ storage/myisam/myisampack
 storage/myisam/rt_test
 storage/myisam/sp_test
 storage/rocksdb/ldb
+storage/rocksdb/myrocks_hotbackup
 storage/rocksdb/mysql_ldb
 storage/rocksdb/sst_dump
 storage/rocksdb/rdb_source_revision.h

CMakeLists.txt

@@ -334,7 +334,8 @@ MYSQL_CHECK_SSL()
 MYSQL_CHECK_READLINE()
 
 SET(MALLOC_LIBRARY "system")
-SET(PYTHON_SHEBANG "/usr/bin/env python")
+SET(PYTHON_SHEBANG "/usr/bin/env python" CACHE STRING "python shebang")
+MARK_AS_ADVANCED(PYTHON_SHEBANG)
 
 CHECK_PCRE()