cppcms: fix build with ICU enabled, but CPPCMS_ICU disabled

cppcms: fix build with ICU enabled, but CPPCMS_ICU disabled

When the ICU package is enabled (BR2_PACKAGE_ICU=y) but ICU support in
cppcms is disabled (BR2_PACKAGE_CPPCMS_ICU is disabled), cppcms still
detects ICU, but it fails to build since we don't get the proper ICU
CXXFLAGS.

In addition, this is not the intended behavior: when
BR2_PACKAGE_CPPCMS_ICU is disabled, we expect cppcms to be built
without ICU support.

To achieve this, we pass -DDISABLE_ICU_LOCALE=ON when
BR2_PACKAGE_CPPCMS_ICU is disabled (yes DISABLE_ICU_LOCALE means the
logic is inverted).

While at it, we pass DISABLE_ICU_LOCALE=OFF when ICU support is
enabled, and not DISABLE_ICONV=ON. The latter has been passed in this
condition since the cppcms package was introduced in Buildroot, but it
doesn't seem to make sense.

Fixes:

  http://autobuild.buildroot.net/results/b6a8c7000c5742efc9d8e13507dab86e36b62840/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>