Add installation hint for Solaris 10

[Bruno Haible]

On Solaris 10 with cc, I get these compilation errors:

source='jitterc/jitterc-generate.c' 
object='jitterc/bin_jitter-jitterc-generate.o' libtool=no \
DEPDIR=.deps depmode=none /bin/bash ../../jitter/build-aux/depcomp \
cc -xarch=generic64 -O -DHAVE_CONFIG_H -I. -I../../jitter -I./config-private 
-I./jitter  -I./gnulib-local -I../../jitter/gnulib-local 
-DJITTER_WITH_LIBTEXTSTYLE=1  
-DJITTER_FLAGDIR=/home/haible/poke-0.90/build-x86-64-cc/jitter/flags  
-DJITTER_TEMPLATEDIR=/home/haible/poke-0.90/build-x86-64-cc/jitter/../../jitter/templates
  -DJITTER_INTERNAL=1 -I/home/haible/prefix-x86_64/include  -g -c -o 
jitterc/bin_jitter-jitterc-generate.o `test -f 'jitterc/jitterc-generate.c' || 
echo '../../jitter/'`jitterc/jitterc-generate.c
"../../jitter/jitterc/jitterc-vm.h", line 450: warning: unnamed struct member
"../../jitter/jitterc/jitterc-rewrite.h", line 214: warning: unnamed union 
member
"../../jitter/jitterc/jitterc-rewrite.h", line 215: warning: unnamed struct 
member
"../../jitter/jitter/jitter-instruction.h", line 124: warning: unnamed union 
member
"../../jitter/jitter/jitter-instruction.h", line 136: warning: unnamed struct 
member
"../../jitter/jitterc/jitterc-generate.c", line 806: improper member use: 
constant
"../../jitter/jitterc/jitterc-generate.c", line 812: improper member use: 
constant
"../../jitter/jitterc/jitterc-generate.c", line 817: improper member use: 
placeholder
"../../jitter/jitterc/jitterc-generate.c", line 820: improper member use: 
placeholder
"../../jitter/jitterc/jitterc-generate.c", line 826: improper member use: 
operator
"../../jitter/jitterc/jitterc-generate.c", line 831: improper member use: 
operator
"../../jitter/jitterc/jitterc-generate.c", line 832: improper member use: 
operator
"../../jitter/jitterc/jitterc-generate.c", line 836: improper member use: 
operand_expressions
"../../jitter/jitterc/jitterc-generate.c", line 836: improper member use: 
operand_expressions
"../../jitter/jitterc/jitterc-generate.c", line 836: improper member use: 
operand_expressions
"../../jitter/jitterc/jitterc-generate.c", line 839: improper member use: 
operand_expressions
"../../jitter/jitterc/jitterc-generate.c", line 1172: improper member use: 
nonresidual_literal
"../../jitter/jitterc/jitterc-generate.c", line 1179: improper member use: 
nonresidual_register
"../../jitter/jitterc/jitterc-generate.c", line 2312: improper member use: 
nonresidual_register
"../../jitter/jitterc/jitterc-generate.c", line 2324: improper member use: 
nonresidual_literal
cc: acomp failed for ../../jitter/jitterc/jitterc-generate.c
*** Error code 2

The cause is that this compiler does not grok unnamed struct/union members.

As I understand from
<https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Unnamed-Fields.html>
this compiler feature exists only in GCC and compatible compilers and in
compilers that implement ISO C11 or newer.

Here's a proposed documentation patch.

0001-Add-installation-hint-for-Solaris-10.patch
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