message: Makefile: option to set install dir
diff: @@ -40,6 +40,8 @@ ROOT_DIR := $(shell dirname $(realpath $(lastword $(MAKEFILE_LIST))))
  # Set default cmake here but replace with special version for Android build.
  CMAKE_BIN = cmake
  +INSTALL_PREFIX ?= $(CURRENT_DIR)/install
  +
  # Function to create build_* directory and call make there.
  define cmake-build
  +@$(eval BUILD_DIR = build/$@)
  @@ -50,7 +52,7 @@ define cmake-build
  && $(CMAKE_BIN) $(ROOT_DIR) $(1) \
  -DEXTERNAL_DIR:STRING=$(EXTERNAL_DIR) \
  -DCMAKE_BUILD_TYPE=$(BUILD_TYPE) \
  - -DCMAKE_INSTALL_PREFIX=$(CURRENT_DIR)/install \
  + -DCMAKE_INSTALL_PREFIX=$(INSTALL_PREFIX) \
  -DDROP_DEBUG=$(DROP_DEBUG) \
  -G$(CMAKE_GENERATOR)) \
  || (rm -rf $(BUILD_DIR)) \
  