[Bug libgcc/81775] New: GCC fails to compile: md-unwind-support.h:65 dereferencing pointer to incomplete type ‘struct ucontext’

["guido at trentalancia dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81775

            Bug ID: 81775
           Summary: GCC fails to compile: md-unwind-support.h:65
                    dereferencing pointer to incomplete type ‘struct
                    ucontext’
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgcc
          Assignee: unassigned at gcc dot gnu.org
          Reporter: guido at trentalancia dot com
  Target Milestone: ---

GCC 7.1.0 fails to rebuild (with the same version of the compiler) because of
the following error:

/home/guido/build-gcc-7.1.0/./gcc/xgcc -B/home/guido/build-gcc-7.1.0/./gcc/
-B/usr/x86_64-pc-linux-gnu/bin/ -B/usr/x86_64-pc-linux-gnu/lib/ -isystem
/usr/x86_64-pc-linux-gnu/include -isystem /usr/x86_64-pc-linux-gnu/sys-include 
  -g -O2 -O3 -march=nehalem -mtune=nehalem -O2  -g -O2 -O3 -march=nehalem
-mtune=nehalem -DIN_GCC    -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual
-Wno-format -Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition 
-isystem ./include   -fpic -mlong-double-80 -DUSE_ELF_SYMVER -g -DIN_LIBGCC2
-fbuilding-libgcc -fno-stack-protector   -fpic -mlong-double-80
-DUSE_ELF_SYMVER -I. -I. -I../.././gcc -I../../../gcc-7.1.0/libgcc
-I../../../gcc-7.1.0/libgcc/. -I../../../gcc-7.1.0/libgcc/../gcc
-I../../../gcc-7.1.0/libgcc/../include
-I../../../gcc-7.1.0/libgcc/config/libbid -DENABLE_DECIMAL_BID_FORMAT
-DHAVE_CC_TLS  -DUSE_TLS -o unwind-dw2-fde-dip.o -MT unwind-dw2-fde-dip.o -MD
-MP -MF unwind-dw2-fde-dip.dep -fexceptions -c
../../../gcc-7.1.0/libgcc/unwind-dw2-fde-dip.c -fvisibility=hidden
-DHIDE_EXPORTS
/home/guido/build-gcc-7.1.0/./gcc/xgcc -B/home/guido/build-gcc-7.1.0/./gcc/
-B/usr/x86_64-pc-linux-gnu/bin/ -B/usr/x86_64-pc-linux-gnu/lib/ -isystem
/usr/x86_64-pc-linux-gnu/include -isystem /usr/x86_64-pc-linux-gnu/sys-include 
  -g -O2 -O3 -march=nehalem -mtune=nehalem -O2  -g -O2 -O3 -march=nehalem
-mtune=nehalem -DIN_GCC    -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual
-Wno-format -Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition 
-isystem ./include   -fpic -mlong-double-80 -DUSE_ELF_SYMVER -g -DIN_LIBGCC2
-fbuilding-libgcc -fno-stack-protector   -fpic -mlong-double-80
-DUSE_ELF_SYMVER -I. -I. -I../.././gcc -I../../../gcc-7.1.0/libgcc
-I../../../gcc-7.1.0/libgcc/. -I../../../gcc-7.1.0/libgcc/../gcc
-I../../../gcc-7.1.0/libgcc/../include
-I../../../gcc-7.1.0/libgcc/config/libbid -DENABLE_DECIMAL_BID_FORMAT
-DHAVE_CC_TLS  -DUSE_TLS -o unwind-sjlj.o -MT unwind-sjlj.o -MD -MP -MF
unwind-sjlj.dep -fexceptions -c ../../../gcc-7.1.0/libgcc/unwind-sjlj.c
-fvisibility=hidden -DHIDE_EXPORTS
In file included from ../../../gcc-7.1.0/libgcc/unwind-dw2.c:403:0:
./md-unwind-support.h: In function ‘x86_64_fallback_frame_state’:
./md-unwind-support.h:65:47: error: dereferencing pointer to incomplete type
‘struct ucontext’
       sc = (struct sigcontext *) (void *) &uc_->uc_mcontext;
                                               ^~
make[3]: *** [../../../gcc-7.1.0/libgcc/shared-object.mk:14: unwind-dw2.o]
Error 1
make[3]: exiting from directory
"/home/guido/build-gcc-7.1.0/x86_64-pc-linux-gnu/libgcc"