make SDK_BASE="/opt/Espressif/ESP8266_SDK" FLAVOR="release" all
I put this line into the Makefile at line 168 to see what compiler command was actually being generated:
$(vecho) "Compiler CMD [$(Q) $(CC) $(INCDIR) $(MODULE_INCDIR) $(EXTRA_INCDIR) $(SDK_INCDIR) $(CFLAGS)"
That showed me that it was attempting to include directories that did not exist, which showed me in turn that my /opt/Espressif/ESP8266_SDK/ directory was not setup for "make SDK_BASE=/opt/Espressif/ESP8266_SDK". I had the espressif SDK in a directory path like this: /opt/Espressif/ESP8266_SDK/esp_iot_sdk_v0.9.5. To make the compile work I had to invoke make like this:
make SDK_BASE="/opt/Espressif/ESP8266_SDK/esp_iot_sdk_v0.9.5" FLAVOR="release" all
Then the compile worked perfectly. Hope this helps. Marshall