Update cmake rules for installing like other cmake projects

[weegreenblobbie]

I can't install cactus-rt into a prefix with cmake, please update the cmake rules to install headers and libs to a configured prefix.

For example, I use the Catch2 C++ testing framework and I can build and install it with this make rule:

$(CATCH_LIB): $(CATCH_REPO)
	@echo "$(BOLD)$(CYAN)Configuring$(RESET) $@"
	$(SILENT)mkdir -p catch2_build && \
	cmake -S Catch2 -B catch2_build \
            -DCMAKE_INSTALL_PREFIX="$(CWD)" \
            -DCATCH_BUILD_TESTING=OFF \
            -DBUILD_SHARED_LIBS=ON $(DEVNULL) && \
	echo "$(BOLD)$(GREEN)Building$(RESET) $@" && \
	cmake --build catch2_build -j $$(nproc) $(DEVNULL) && \
	echo "$(BOLD)$(CYAN)Installing$(RESET) $@" && \
	cmake --install catch2_build $(DEVNULL)

I'd love to follow the same pattern for cactus-rt!