Avoid hardcoding /usr/local/bin/clang70 as fallback linker

--- Makefile-ponyc.orig	2019-11-02 05:19:12 UTC
+++ Makefile-ponyc
@@ -130,7 +130,7 @@ LINKER_FLAGS = -march=$(arch) -mtune=$(tune) $(LDFLAGS
 AR_FLAGS ?= rcs
 ALL_CFLAGS = -std=gnu11 -fexceptions \
   -DPONY_VERSION=\"$(tag)\" -DLLVM_VERSION=\"$(llvm_version)\" \
-  -DPONY_COMPILER=\"$(CC)\" -DPONY_ARCH=\"$(arch)\" \
+  -DPONY_COMPILER=\"/usr/bin/cc\" -DPONY_ARCH=\"$(arch)\" \
   -DBUILD_COMPILER=\"$(compiler_version)\" \
   -DPONY_BUILD_CONFIG=\"$(config)\" \
   -DPONY_VERSION_STR=\"$(version_str)\" \