Fix use of a nonexistent CMake variable

SAMPGDK_CURRENT_BINARY_DIR doesn't exist and hence is empty, otherwise this
just wouldn't work.
Revert "Make generated source files self-contained"

This reverts commit 6e528b9f17e6e4d6a36d9b3378b805904bf30ffa.

Conflicts:
	CMakeLists.txt
	src/a_objects.c
	src/a_samp.c
	src/a_vehicles.c
	src/codegen.py
	src/internal/amx.c