The package name is "libsecret-1". You can use it like this in your Makefile.am file:

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AM_VALAFLAGS = \
    --pkg=libsecret-1

Some parts of the libsecret API are not yet stable. To use them you need to define the SECRET_WITH_UNSTABLE C preprocessor macro to use them, or else the build will fail:

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AM_CPPFLAGS = \
    -DSECRET_WITH_UNSTABLE=1