jemalloc-sys: version macro being 0

The jemalloc.h produced by jemalloc-sys contain the following macros:

#define JEMALLOC_VERSION "0.0.0-0-g0000000000000000000000000000000000000000"
#define JEMALLOC_VERSION_MAJOR 0
#define JEMALLOC_VERSION_MINOR 0
#define JEMALLOC_VERSION_BUGFIX 0
#define JEMALLOC_VERSION_NREV 0
#define JEMALLOC_VERSION_GID "0000000000000000000000000000000000000000"

where for example JEMALLOC_VERSION_MAJOR should be 5 (for jemalloc 5.1.0). This will break c/c++ code that use the header to detect jemalloc version. Reproducible on master and jemalloc-sys:0.3.2. It was fine with jemalloc-sys:1.7.0.