Only add -stdlib=xxxxx flags if they are not already defined

Makefile

@@ -10,15 +10,20 @@ OBJCONV = objconv
 
 CPPFLAGS_add = -I$(LIBOSXUNWIND_HOME)/src -I$(LIBOSXUNWIND_HOME)/include -DNDEBUG
 CFLAGS_add = -std=c99 -Wall -O3
+CXXFLAGS_add = -std=c++11 -Wall -O3
+LDFLAGS_add = -nodefaultlibs -Wl,-upward-lSystem -Wl,-umbrella,System
 SFLAGS_add = -x assembler-with-cpp
 
+# If `-stdlib=` is specified within our environment variables, then don't add another command line argument asking to link against it..
+ifneq (,$(findstring -stdlib=,$(CC) $(CPPFLAGS) $(CXXFLAGS)))
+# If our clang is new enough, then we need to add in `-stdlib=libstdc++`.
 CLANG_MAJOR_VER := $(shell clang -v 2>&1 | grep LLVM | cut -d' ' -f 4 | cut -d'.' -f 1)
 ifeq ($(shell [ $(CLANG_MAJOR_VER) -ge 8 ] && echo true),true)
-CXXFLAGS_add = -std=c++11 -stdlib=libstdc++ -Wall -O3
-LDFLAGS_add = -nodefaultlibs -stdlib=libstdc++ -Wl,-upward-lSystem -Wl,-umbrella,System
+CXXFLAGS_add += -stdlib=libstdc++
+LDFLAGS_add += -stdlib=libstdc++
 else
-CXXFLAGS_add = -std=c++11 -Wall -O3
-LDFLAGS_add = -nodefaultlibs -Wl,-upward-lSystem -Wl,-umbrella,System -lstdc++
+LDFLAGS_add += -lstdc++
+endif
 endif
 
 # Files (in src/)