fix: build library without solana sdk

Author: Riateche

program/c/makefile

@@ -3,12 +3,14 @@
 OUT_DIR := $(if $(OUT_DIR),$(OUT_DIR),./target)
 SOLANA := $(shell dirname $(shell which cargo-build-bpf))
 
-ifneq ("$(wildcard $(SOLANA)/sdk/bpf/c/bpf.mk)","")
-  $(info using Solana BPF SDK)
-  include $(SOLANA)/sdk/bpf/c/bpf.mk
-else
-  $(info using Solana SBF SDK)
-  include $(SOLANA)/sdk/sbf/c/sbf.mk
+ifneq ("$(SOLANA)","")
+  ifneq ("$(wildcard $(SOLANA)/sdk/bpf/c/bpf.mk)","")
+    $(info using Solana BPF SDK)
+    include $(SOLANA)/sdk/bpf/c/bpf.mk
+  else
+    $(info using Solana SBF SDK)
+    include $(SOLANA)/sdk/sbf/c/sbf.mk
+  endif
 endif
 
 FEATURES_H_BODY:="\#pragma once"