build failure on Solaris 8 with GCC 4.5.1

Here are the symptoms:

gcc -std=gnu99 -std=gnu99 -DHAVE_CONFIG_H -I. -I../lib  -I'../lib'   -g -O2 -MT b-fro.o -MD -MP -MF .deps/b-fro.Tpo -c -o b-fro.o b-fro.c
In file included from b-fro.c:37:
b-divvy.h:48:1: warning: "SHARED" redefined
In file included from b-fro.c:32:
/usr/include/sys/mman.h:81:1: warning: this is the location of the previous definition
...


gcc -std=gnu99 -std=gnu99 -DHAVE_CONFIG_H -I. -I../lib  -I'../lib'   -g -O2 -MT b-isr.o -MD -MP -MF .deps/b-isr.Tpo -c -o b-isr.o b-isr.c
b-isr.c: In function 'complain_signal':
b-isr.c:84: warning: implicit declaration of function 'psignal'
b-isr.c: In function 'scratch_from_context':
b-isr.c:139: error: 'ucontext_t' undeclared (first use in this function)
b-isr.c:139: error: (Each undeclared identifier is reported only once
b-isr.c:139: error: for each function it appears in.)
b-isr.c:139: error: 'uc' undeclared (first use in this function)
b-isr.c: In function 'catchsigaction':
b-isr.c:183: warning: implicit declaration of function 'psiginfo'


The latter objections were fatal: the compilation failed.