Similar to the thread regarding build failures w/ DO_XSI_MATH disabled, I think there is another breakage with the new FENV bits that were added by commit ea38f4d89c96, which when built natively on an existing uclibc-ng-1.0.26 chroot yield these failures:
LD libuClibc-0.1.0.27.so libc/libc_so.a(w_acos.os): In function `__GI_acos': w_acos.c:(.text+0x74): undefined reference to `feraiseexcept' libc/libc_so.a(k_standardl.os): In function `__kernel_standard_l': k_standardl.c:(.text+0x10): undefined reference to `feholdexcept' k_standardl.c:(.text+0x70): undefined reference to `fesetenv' collect2: error: ld returned 1 exit status make: *** [libc/Makefile.in:77: lib/libc.so] Error 1
I am not sure why these symbols go missing. Possibly something related to kernel headers? My chroot environment has kernel headers from 4.13 installed, so I am thinking this isn't the issue. I have both DO_C99_MATH and DO_XSI_MATH enabled, using legacy NAN (not 2008), and various other glibc-compatibility tweaks enabled. uclibc-ng-1.0.26 built fine -- the only real issue there was my previously-reported xfsprogs build failure.
I also read earlier that it is considered uncommon to build uclibc-ng natively. I'd like to call that into question and recommend that native builds be tested in some capacity before new releases are made. An existing environment should be capable of upgrading itself.