cmake: Use ${CMAKE_COMMAND} instead of `cmake`

Not all users have `cmake` on their PATH and this causes a build
failure.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index b0b2fef..e02849d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -138,7 +138,7 @@
 		OUTPUT build/browse_py.h
 		MAIN_DEPENDENCY src/browse.py
 		DEPENDS src/inline.sh
-		COMMAND cmake -E make_directory ${CMAKE_BINARY_DIR}/build
+		COMMAND ${CMAKE_COMMAND} -E make_directory ${CMAKE_BINARY_DIR}/build
 		COMMAND src/inline.sh kBrowsePy
 						< src/browse.py
 						> ${CMAKE_BINARY_DIR}/build/browse_py.h