Tweaks to the ci plugin after trying CircleCI

For best results use 'make -k ci'. 'make ci-prepare' can be
used to setup the OTP installations needed.

plugins/ci.mk

@@ -16,7 +16,9 @@ CI_OTP ?=
 ifeq ($(strip $(CI_OTP)),)
 ci::
 else
-ci:: $(KERL) $(addprefix ci-,$(CI_OTP))
+ci:: $(addprefix ci-,$(CI_OTP))
+
+ci-prepare: $(addprefix $(CI_INSTALL_DIR)/,$(CI_OTP))
 
 ci-setup::
 
@@ -25,7 +27,7 @@ ci_verbose = $(ci_verbose_$(V))
 
 define ci_target
 ci-$(1): $(CI_INSTALL_DIR)/$(1)
-	-$(ci_verbose) \
+	$(ci_verbose) \
 		PATH="$(CI_INSTALL_DIR)/$(1)/bin:$(PATH)" \
 		CI_OTP_RELEASE="$(1)" \
 		CT_OPTS="-label $(1)" \
@@ -35,9 +37,11 @@ endef
 $(foreach otp,$(CI_OTP),$(eval $(call ci_target,$(otp))))
 
 define ci_otp_target
-$(CI_INSTALL_DIR)/$(1):
+ifeq ($(wildcard $(CI_INSTALL_DIR)/$(1)),)
+$(CI_INSTALL_DIR)/$(1): $(KERL)
 	$(KERL) build git $(OTP_GIT) $(1) $(1)
 	$(KERL) install $(1) $(CI_INSTALL_DIR)/$(1)
+endif
 endef
 
 $(foreach otp,$(CI_OTP),$(eval $(call ci_otp_target,$(otp))))