Use label syntax that is immune to MSYS path mangling in bootstrap scripts.

On Windows, MSYS will mangle all arguments that resemble Unix paths
when executing (exec*()) non-msys executables (in an attempt to convert
them to Windows paths). This affects ``//src:bazel`` (it becomes
``/src:bazel``) but not ``src:bazel``. This CL converts to the latter in
bootstrapping shell scripts to work around this issue.

--
MOS_MIGRATED_REVID=113349821

compile.sh

diff --git a/compile.sh b/compile.sh
index 9f435e0..880555d 100755
--- a/compile.sh
+++ b/compile.sh
@@ -89,7 +89,7 @@
   new_step 'Building Bazel with Bazel'
   display "."
   log "Building output/bazel"
-  bazel_build //src:bazel
+  bazel_build src:bazel
   cp -f bazel-bin/src/bazel output/bazel
   chmod 0755 output/bazel
   BAZEL=$(pwd)/output/bazel

scripts/bootstrap/bootstrap.sh

diff --git a/scripts/bootstrap/bootstrap.sh b/scripts/bootstrap/bootstrap.sh
index 16a13c9..13e71c0 100755
--- a/scripts/bootstrap/bootstrap.sh
+++ b/scripts/bootstrap/bootstrap.sh
@@ -77,7 +77,7 @@
 function bootstrap_test() {
   local BAZEL_BIN=$1
   local BAZEL_SUM=$2
-  local BAZEL_TARGET=${3:-//src:bazel}
+  local BAZEL_TARGET=${3:-src:bazel}
   [ -x "${BAZEL_BIN}" ] || fail "syntax: bootstrap bazel-binary"
   run_silent ${BAZEL_BIN} --nomaster_bazelrc --bazelrc=${BAZELRC} clean \
       --expunge || return $?