I just found out that Ruby 1.8 gives a more descriptive error
message:
===================================
$ sudo LANG=en_US.utf8 gem18 install ffi
Building native extensions.  This could take a while...
ERROR:  Error installing ffi:
	ERROR: Failed to build gem native extension.

        /opt/rubies/ruby-1.8.7-p334/bin/ruby18 extconf.rb
checking for ffi_call() in -lffi... yes
checking for ffi_prep_closure()... yes
checking for ffi_raw_call()... yes
checking for ffi_prep_raw_closure()... yes
checking for rb_thread_blocking_region()... no
checking for ruby_thread_has_gvl_p()... no
checking for ruby_native_thread_p()... no
checking for rb_thread_call_with_gvl()... no
creating extconf.h
creating Makefile

make
gcc -I. -I. -I/opt/rubies/ruby-1.8.7-p334/lib/ruby/1.8/x86_64-linux -
I. -DRUBY_EXTCONF_H=\"extconf.h\"    -fPIC -g -O2  -fPIC  -I/usr/lib/
libffi-3.0.9/include   -Wno-declaration-after-statement   -c
DataConverter.c
In file included from /usr/include/linux/param.h:4:0,
                 from /usr/include/sys/param.h:28,
                 from ./endian.h:4,
                 from /usr/include/bits/waitstatus.h:65,
                 from /usr/include/stdlib.h:43,
                 from /opt/rubies/ruby-1.8.7-p334/lib/ruby/1.8/x86_64-
linux/ruby.h:40,
                 from DataConverter.c:2:
/usr/include/asm/param.h:1:31: fatal error: asm-generic/param.h: No
such file or directory
compilation terminated.
make: *** [DataConverter.o] Error 1


Gem files will remain installed in /opt/rubies/ruby-1.8.7-p334/lib/
ruby/gems/1.8/gems/ffi-1.0.7 for inspection.
Results logged to /opt/rubies/ruby-1.8.7-p334/lib/ruby/gems/1.8/gems/
ffi-1.0.7/ext/ffi_c/gem_make.out

===================================

The informative part is this one:

/usr/include/asm/param.h:1:31: fatal error: asm-generic/param.h: No
such file or directory

I saw a similar error when I compiled my Ruby, and wondered why
various asm files are missing from my system. Since that day I had to
compile my Ruby with "CFLAGS=-I/usr/src/linux-2.6.37-ARCH/include"
appended to it, which worked but seemed ugly to me. Any clue on what
is broken on my system?

Vale,
Quintus