Take a look at the use of this make variable. Ensure CFLAGS content is correctly distributed. Also, scan the Makefiles for make vs $(MAKE).
https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=f5ae31ebd3d49a064cb85ccd62a75198883cec3b
https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=264b784089bfb56b0039c1ebfc5e92912f0284ce https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=15a3d49bf12b24c0a353525acb93e188f67e7581
This (last) commit should hopefully nail this: https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=2762c2c1a8c98d9012fcd40f20d133493a0b3219