Bootstrapping on AIX 4.2.1 (yeah, I know it's old) fails because of some missing
declarations (FILHDR and SYMENT, they're in filehdr.h syms.h respectively).  I
got it to build by adding those includes to gcc/collect2.c (I'll attach the
trivial patch, it's probably not the right thing but it works for me).

$ /home/dtucker/gcc-3.4.0/configure --enable-languages=c --disable-nls
--with-included-gettext --enable-threads=aix && make bootstrap
[snip build]
gcc   -g  -DIN_GCC   -W -Wall -Wwrite-strings -Wstrict-prototypes
-Wmissing-prototypes -pedantic -Wno-long-long    -DHAVE_CONFIG_H    -I. -I.
-I/home/dtucker/gcc-3.4.0/gcc -I/home/dtucker/gcc-3.4.0/gcc/.
-I/home/dtucker/gcc-3.4.0/gcc/../include   \
-DTARGET_MACHINE=\"powerpc-ibm-aix4.2.1.0\" \
-c /home/dtucker/gcc-3.4.0/gcc/collect2.c -o collect2.o
In file included from /home/dtucker/gcc-3.4.0/gcc/collect2.c:102:
/usr/include/ldfcn.h:50: error: parse error before "FILHDR"
/usr/include/ldfcn.h:52: error: parse error before '}' token
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2645: error: parse error before "SYMENT"
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2645: warning: function declaration isn't
a prototype
/home/dtucker/gcc-3.4.0/gcc/collect2.c: In function `scan_prog_file':
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2684: error: dereferencing pointer to
incomplete type
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2684: error: `U802WRMAGIC' undeclared
(first use in this function)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2684: error: (Each undeclared identifier
is reported only once
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2684: error: for each function it appears
in.)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2684: error: dereferencing pointer to
incomplete type
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2684: error: `U802ROMAGIC' undeclared
(first use in this function)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2684: error: dereferencing pointer to
incomplete type
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2684: error: `U802TOCMAGIC' undeclared
(first use in this function)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2684: error: dereferencing pointer to
incomplete type
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2684: error: dereferencing pointer to
incomplete type
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2684: error: dereferencing pointer to
incomplete type
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2687: error: dereferencing pointer to
incomplete type
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2687: error: dereferencing pointer to
incomplete type
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2689: error: dereferencing pointer to
incomplete type
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2694: error: dereferencing pointer to
incomplete type
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2694: error: `F_SHROBJ' undeclared (first
use in this function)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2699: error: `SYMENT' undeclared (first
use in this function)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2699: error: parse error before "symbol"
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2701: warning: implicit declaration of
function `ldtbread'
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2701: error: `symbol' undeclared (first
use in this function)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2705: error: `C_EXT' undeclared (first
use in this function)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2705: error: `N_UNDEF' undeclared (first
use in this function)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2705: error: `N_TMASK' undeclared (first
use in this function)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2705: error: `DT_NON' undeclared (first
use in this function)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2705: error: `N_BTSHFT' undeclared (first
use in this function)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2705: error: `DT_FCN' undeclared (first
use in this function)
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2799: error: dereferencing pointer to
incomplete type
/home/dtucker/gcc-3.4.0/gcc/collect2.c:2810: warning: implicit declaration of
function `ldclose'