build: fix CMAKE_MODULE_PATH usage

Fixes #1447.  `CMAKE_MODULE_PATH` is meant to be a list of directories,
and as such, is not the proper way to launch our scripts.  Let's use
`${PROJECT_SOURCE_DIR}/cmake` instead.  Also, let's not outright set
`CMAKE_MODULE_PATH`, but instead append our location to the list.
Merge pull request #1449 from jszakmeister/fix-cmake-module-path-usage

build: fix CMAKE_MODULE_PATH usage