prepare before build

prepare needs to run before build, otherwise the following error occurs on a pristine clone:

```
make -e -C /home/bene/projects/remerge/aerospike-client-c/modules/base libaerospike-base.a COMMON=/home/bene/projects/remerge/aerospike-client-c/modules/common
make[1]: Entering directory `/home/bene/projects/remerge/aerospike-client-c/modules/base'
cc -Isrc/include -I/home/bene/projects/remerge/aerospike-client-c/modules/common/target/Linux-x86_64/include -std=gnu99 -g -rdynamic -Wextra -fno-common -fno-strict-aliasing -fPIC -O3 -D_FILE_OFFSET_BITS=64 -D_REENTRANT -D_GNU_SOURCE -o target/Linux-x86_64/obj/base/cf_hist.o -c src/main/cf_hist.c
src/main/cf_hist.c:32:32: fatal error: citrusleaf/cf_bits.h: No such file or directory
compilation terminated.
make[1]: *** [target/Linux-x86_64/obj/base/cf_hist.o] Error 1
make[1]: Leaving directory `/home/bene/projects/remerge/aerospike-client-c/modules/base'
make: *** [/home/bene/projects/remerge/aerospike-client-c/modules/base/target/Linux-x86_64/lib/libaerospike-base.a] Error 2
```

i've also added the standard CFLAGS environment variable to the Makefile.