support/dependencies: ensure we have 'file' on the host

Recently, the autoconf macros for libtool started using '/usr/bin/file'
to determine the type of library that is generated by the toolchain.
Packages that use this recent version of the libtool autoconf macros
will fail in a rather dramatic way when /usr/bin/file is not present
on the host: the package will still build but no shared library is
generated, which in turn may cause build failures in other packages
that link with it.

For example, libpng's configure determines that it is not possible to
build a shared library on MIPS64 because the expected output from 'file'
is not present. Therefore, only a static libpng.a is built. Later,
bandwithd links with -lpng but it doesn't use the pkg-config's
Private-Libs (because it's not linking statically) and it doesn't have
access to the NEEDED reference from the shared library. Therefore, it
doesn't link with zlib and fails with

    pngrutil.c:(.text+0x55c): undefined reference to `inflate'

We cant use host-file because it is itself an autotools package and is
itself using libtool, so this would be a chicken-n-egg problem. Besides,
the libtool script really wants to call /usr/bin/file, so it would not
even find our host-file anyway.

So, just require that '/usr/bin/file' is present on the host.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/prerequisite.txt

@@ -35,6 +35,7 @@ between distributions).
 ** +python+ (version 2.6 or any later)
 ** +unzip+
 ** +rsync+
+** +file+ (must be in +/usr/bin/file+)
 
 * Source fetching tools:
 ** +wget+

support/dependencies/dependencies.sh

@@ -69,6 +69,10 @@ check_prog_host "which"
 # Verify that sed is installed
 check_prog_host "sed"
 
+# 'file' must be present and must be exactly /usr/bin/file,
+# otherwise libtool fails in incomprehensible ways.
+check_prog_host "/usr/bin/file"
+
 # Check make
 MAKE=$(which make 2> /dev/null)
 if [ -z "$MAKE" ] ; then