package/erlang: add optional dependency to unixodbc

package/erlang: add optional dependency to unixodbc

Quote from erlang configure without unixodbc:

checking for odbc in standard locations... no
configure: WARNING: No odbc library found skipping odbc
configure: WARNING: "ODBC library - header check failed"
configure: WARNING: "ODBC library - link check failed"

Quote from erlang configure with unixodbc:

checking for odbc in standard locations... -L/home/buildroot/br3_couchdb/output/host/usr/i586-buildroot-linux-uclibc/sysroot/usr/lib
checking for SQLAllocHandle in -lodbc... yes

To check for unixodbc support used by erlang use readelf:

$ output/host/usr/bin/i586-buildroot-linux-uclibc-readelf -a output/target/usr/lib/erlang/lib/odbc-2.11.1/priv/bin/odbcserver | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libodbc.so.2]
 0x00000001 (NEEDED)                     Shared library: [libc.so.0]

Tested using this minimal defconfig:

BR2_PACKAGE_ERLANG=y
BR2_PACKAGE_ERLANG_SMP=y
BR2_PACKAGE_UNIXODBC=y

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Tested-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>