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

--- Makefile-ponyc.orig	2020-02-03 03:34:35 UTC
+++ Makefile-ponyc
@@ -146,13 +146,13 @@ else
 endif
 
 LIB_EXT ?= a
-BUILD_FLAGS = -march=$(arch) -mtune=$(tune) -Werror -Wconversion \
+BUILD_FLAGS = -march=$(arch) -mtune=$(tune) -Wconversion \
   -Wno-sign-conversion -Wextra -Wall
 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)\" \