Stop continually re-creating build_version.c

Summary:
We continually rebuilt build_version.c because we put the
current date into it, but that's what __DATE__ already is.  This makes
builds faster.

This also fixes an issue with 'make clean FOO' not working properly.

Also tweak the build rules to be more consistent, always have warnings,
and add a 'make release' rule to handle flags for release builds.

Test Plan: make, make clean

Reviewers: dhruba

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D8139

build_detect_version

@@ -8,7 +8,8 @@
 #
 
 # create git version file
-VFILE=util/build_version.cc
+VFILE=$(mktemp)
+trap "rm $VFILE" EXIT
 
 # check to see if git is in the path
 which git > /dev/null
@@ -19,6 +20,10 @@ else
   echo "git not found"| awk ' BEGIN {print "#include \"build_version.h\""} {print "const char * leveldb_build_git_sha = \"leveldb_build_git_sha:git not found\";"} END {}' > ${VFILE}
 fi
 
-date | awk 'BEGIN {} {print "const char * leveldb_build_git_datetime = \"leveldb_build_git_datetime:"$0"\";"} END {} ' >> ${VFILE}
 echo "const char * leveldb_build_compile_date = __DATE__;" >> ${VFILE}
 echo "const char * leveldb_build_compile_time = __TIME__;" >> ${VFILE}
+
+OUTFILE=util/build_version.cc
+if [ ! -e $OUTFILE ] || ! cmp -s $VFILE $OUTFILE; then
+    cp $VFILE $OUTFILE
+fi