ghc-8.10.1-alpha1 fails to build with --with-system-libffi --with-ffi-includes= when built with ghc-8.6.5

Gentoo has libffi headers in a location not seen by compiler by default, for example in /usr/lib64/libffi-3.3/include.

Build attempt against external libffi fails as:

$ ./configure --with-system-libffi --with-ffi-includes=/usr/lib64/libffi-3.3/include
...
$ make
...
x86_64-pc-linux-gnu-gcc -E  -DPROFILING -DTHREADED_RTS -DDEBUG   -march=sandybridge -mtune=sandybridge -O2 -pipe -fdiagnostics-show-option -frecord-gcc-switches -Wall -Wextra -Wstack-protector -g -march=sandybridge -march=sandybridge -mtune=sandybridge -mtune=sandybridge -g -Wall  -Wall -Wextra -Wstrict-prototypes  -Wmissing-prototypes  -Wmissing-declarations -Winline -Wpointer-arith -Wmissing-noreturn -Wnested-externs -Wredundant-decls -Wno-aggregate-return -Iincludes -Iincludes/dist -Iincludes/dist-derivedconstants/header -Iincludes/dist-ghcconstants/header -Iincludes/dist-install/build -Irts -Irts/dist/build -DCOMPILING_RTS -DFS_NAMESPACE=rts -fno-strict-aliasing -fno-common -Irts/dist/build/./autogen        -Werror=unused-but-set-variable -Wno-error=inline -O2 -fomit-frame-pointer -g -DRtsWay=\"rts_v\"    -ffunction-sections -fdata-sections -Wno-strict-prototypes -MM -x c rts/Interpreter.c -MF rts/dist/build/.depend-v-dyn-l-debug-thr-thr_debug-thr_l-debug_dyn-thr_dyn-thr_debug_dyn-l_dyn-thr_l_dyn.c_asm.bit
rts/Interpreter.c:41:10: fatal error: ffi.h: No such file or directory
   41 | #include "ffi.h"
      |          ^~~~~~~

Reverting commit 3ce18700 restores ability to boot with ghc-8.6.5.

I guess these flags will become redundant only after lower booter ghc version is guaranteed to ember libffi flags.