fixups for recent "Cleanup jenkins build scripts"

Change I24e500e132f5c8e8133d35548cb7b4e4552331d0 was merged, but IMHO needs
improvement. Fix these:

- clean the git source tree before each build step, in common prep_build().
- fix indenting inside the build() macros.
- change build() arg to be build_dir, to absorb cleanup steps into build().
- in jenkins.sh, use $ENABLE_SANITIZE as global env, not passed as arg.
- in jenkins.sh, don't do 'make distcheck' twice. It is not necessary to do it
  from source tree as well as separately from source tree, since distcheck
  already moves to a different build dir.

Change-Id: I09d306350602f21943d5bd45f7388c83ede9b524
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index c397d52..d731504 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -9,19 +9,23 @@
         ENABLE_SANITIZE=""
 fi
 
+src_dir="$PWD"
 build() {
-    $1 --enable-static $2 CFLAGS="-Werror" CPPFLAGS="-Werror"
-$MAKE $PARALLEL_MAKE check \
-  || cat-testlogs.sh
-$MAKE distcheck \
-  || cat-testlogs.sh
+    build_dir="$1"
+
+    prep_build "$src_dir" "$build_dir"
+
+    "$src_dir"/configure --enable-static $ENABLE_SANITIZE CFLAGS="-Werror" CPPFLAGS="-Werror"
+    $MAKE $PARALLEL_MAKE check \
+        || cat-testlogs.sh
 }
 
 # verify build in dir other than source tree
-mkdir -p builddir
-cd builddir
-build ../configure $ENABLE_SANITIZE
+build builddir
+# verify build in source tree
+build .
 
-cd ..
-build ./configure $ENABLE_SANITIZE
-
+# do distcheck only once, which is fine from built source tree, since distcheck
+# is well separated from the source tree state.
+$MAKE distcheck \
+    || cat-testlogs.sh