How to get a shell environment variable in a makefile?

for those who want some official document to confirm the behavior

Variables in make can come from the environment in which make is run. Every environment variable that make sees when it starts up is transformed into a make variable with the same name and value. However, an explicit assignment in the makefile, or with a command argument, overrides the environment. (If the ‘-e’ flag is specified, then values from the environment override assignments in the makefile.

https://www.gnu.org/software/make/manual/html_node/Environment.html

all:
    echo ${PATH}

Or change PATH just for one command:

all:
    PATH=/my/path:${PATH} cmd

If you've exported the environment variable:

export demoPath=/usr/local/demo

you can simply refer to it by name in the makefile (make imports all the environment variables you have set):

DEMOPATH = ${demoPath}    # Or $(demoPath) if you prefer.

If you've not exported the environment variable, it is not accessible until you do export it, or unless you pass it explicitly on the command line:

make DEMOPATH="${demoPath}" …

If you are using a C shell derivative, substitute setenv demoPath /usr/local/demo for the export command.