[bug] forcing build settings on rhel7 without specific checks is not appropriate

Hello,

there was a recent change where a couple of compiler settings are
forced on rhel7:

# centos7/rhel7 provides gcc 4.8.5 and zlib 1.2.7.
ifneq ($(findstring .el7.,$(uname_R)),)
    BASIC_CFLAGS += -std=c99
    NO_UNCOMPRESS2 = YesPlease
endif

I believe this is not the right way to do it: the makefile should
check for the actual compiler and zstd versions and make the decision
based on that. Yocto project uses a much newer (cross)compiler and
zlib for cross-builds, and those settings are not only unnecessary,
but actively harm reproducibility, as cross-binaries built on
rhel7/centos7 are then not the same as binaries built on other
distributions. So we ended up reverting the change:
https://lists.openembedded.org/g/openembedded-core/message/161456

Thanks,
Alex