Change LIBXML_INC and ALLJOYN_INSTALL_DIR in the Makefile to use environment variables if set.

Makefile

@@ -18,10 +18,9 @@ SRCS=DeviceProviders/AllJoynProvider.cpp \
      Adapters/ZigBeeAdapter/ZigBeeAdapter.cpp \
      main.cpp
 
-LIBXML_INC=/usr/include/libxml2
+LIBXML_INC?=/usr/include/libxml2
 
-ALLJOYN_INSTALL_DIR=/Users/jgladi200/Work/alljoyn/alljoyn-15.09.00a-src/build/darwin/x86/debug/dist/cpp
-#ALLJOYN_INSTALL_DIR=/home/gladish/work/alljoyn-15.09.00-src/build/linux/x86_64/debug/dist/cpp/
+ALLJOYN_INSTALL_DIR?=/Users/jgladi200/Work/alljoyn/alljoyn-15.09.00a-src/build/darwin/x86/debug/dist/cpp
 
 CXXFLAGS=-D QCC_OS_GROUP_POSIX -Wall -Wextra -Wno-missing-field-initializers -Wno-deprecated-declarations -g -std=c++0x -I. -I$(ALLJOYN_INSTALL_DIR)/inc -I$(LIBXML_INC)
 CXXFLAGS+=-Wno-ignored-qualifiers