Building the main Guest Additions module for kernel 4.20.0-994-generic.
Error building the module.  Build output follows.
make V=1 CONFIG_MODULE_SIG= -C /lib/modules/4.20.0-994-generic/build SUBDIRS=/tmp/vbox.0 SRCROOT=/tmp/vbox.0 -j1 modules
test -e include/generated/autoconf.h -a -e include/config/auto.conf || (		\
echo >&2;							\
echo >&2 "  ERROR: Kernel configuration is invalid.";		\
echo >&2 "         include/generated/autoconf.h or include/config/auto.conf are missing.";\
echo >&2 "         Run 'make oldconfig && make prepare' on kernel src to fix it.";	\
echo >&2 ;							\
/bin/false)
mkdir -p /tmp/vbox.0/.tmp_versions ; rm -f /tmp/vbox.0/.tmp_versions/*
make -f ./scripts/Makefile.build obj=/tmp/vbox.0
(cat /dev/null;   echo kernel//tmp/vbox.0/vboxguest.ko;) > /tmp/vbox.0/modules.order
  gcc -Wp,-MD,/tmp/vbox.0/.VBoxGuest-linux.o.d  -nostdinc -isystem include -I./arch/x86/include -I./arch/x86/include/generated  -I./include -I./arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/kconfig.h -include ./include/linux/compiler_types.h -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -fshort-wchar -Werror-implicit-function-declaration -Wno-format-security -std=gnu89 -fno-PIE -DCC_HAVE_ASM_GOTO -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -m64 -falign-jumps=1 -falign-loops=1 -mno-80387 -mno-fp-ret-in-387 -mpreferred-stack-boundary=3 -mskip-rax-setup -mtune=generic -mno-red-zone -mcmodel=kernel -funit-at-a-time -DCONFIG_X86_X32_ABI -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_FXSAVEQ=1 -DCONFIG_AS_SSSE3=1 -DCONFIG_AS_AVX=1 -DCONFIG_AS_AVX2=1 -DCONFIG_AS_AVX512=1 -DCONFIG_AS_SHA1_NI=1 -DCONFIG_AS_SHA256_NI=1 -Wno-sign-compare -fno-asynchronous-unwind-tables -mindirect-branch=thunk-extern -mindirect-branch-register -fno-delete-null-pointer-checks -Wno-frame-address -Wno-format-truncation -Wno-format-overflow -Wno-int-in-bool-context -O2 --param=allow-store-data-races=0 -Wframe-larger-than=1024 -fstack-protector-strong -Wno-unused-but-set-variable -Wno-unused-const-variable -fno-omit-frame-pointer -fno-optimize-sibling-calls -fno-var-tracking-assignments -pg -mrecord-mcount -mfentry -DCC_USING_FENTRY -Wdeclaration-after-statement -Wvla -Wno-pointer-sign -fno-strict-overflow -fno-merge-all-constants -fmerge-constants -fno-stack-check -fconserve-stack -Werror=implicit-int -Werror=strict-prototypes -Werror=date-time -Werror=incompatible-pointer-types -Werror=designated-init -Wno-declaration-after-statement -include /tmp/vbox.0/include/VBox/VBoxGuestMangling.h -fno-pie -I/lib/modules/4.20.0-994-generic/build/include -I/tmp/vbox.0/ -I/tmp/vbox.0/include -I/tmp/vbox.0/r0drv/linux -I/tmp/vbox.0/vboxguest/ -I/tmp/vbox.0/vboxguest/include -I/tmp/vbox.0/vboxguest/r0drv/linux -D__KERNEL__ -DMODULE -DVBOX -DRT_OS_LINUX -DIN_RING0 -DIN_RT_R0 -DIN_GUEST -DIN_GUEST_R0 -DIN_MODULE -DRT_WITH_VBOX -DVBGL_VBOXGUEST -DVBOX_WITH_HGCM -DRT_ARCH_AMD64 -DVBOX_WITH_64_BITS_GUESTS  -DMODULE  -DKBUILD_BASENAME='"VBoxGuest_linux"' -DKBUILD_MODNAME='"vboxguest"' -c -o /tmp/vbox.0/VBoxGuest-linux.o /tmp/vbox.0/VBoxGuest-linux.c
In file included from /tmp/vbox.0/include/iprt/types.h:31:0,
                 from /tmp/vbox.0/r0drv/linux/the-linux-kernel.h:34,
                 from /tmp/vbox.0/VBoxGuest-linux.c:36:
/tmp/vbox.0/include/iprt/stdarg.h:44:12: fatal error: stdarg.h: No such file or directory
 #  include <stdarg.h>
            ^~~~~~~~~~
compilation terminated.
scripts/Makefile.build:291: recipe for target '/tmp/vbox.0/VBoxGuest-linux.o' failed
make[2]: *** [/tmp/vbox.0/VBoxGuest-linux.o] Error 1
Makefile:1563: recipe for target '_module_/tmp/vbox.0' failed
make[1]: *** [_module_/tmp/vbox.0] Error 2
/tmp/vbox.0/Makefile.include.footer:101: recipe for target 'vboxguest' failed
make: *** [vboxguest] Error 2
/sbin/ldconfig.real: /usr/lib/i386-linux-gnu/libfontconfig.so.1 is not a symbolic link