package/beecrypt: fix build without C++

package/beecrypt: fix build without C++

Do not check for C++ compiler as C++ support has been disabled since
commit dd4d3c18d6753e1224fbe59d91a4b44f39bc38c0 otherwise
build will fail on toolchains without a working C++ compiler:

checking how to run the C++ preprocessor... /lib/cpp
configure: error: in `/data/buildroot/buildroot-test/instance-1/output/build/beecrypt-4.2.1':
configure: error: C++ preprocessor "/lib/cpp" fails sanity check

Fixes:
 - http://autobuild.buildroot.org/results/3c79cc68f1b088ad24daf7f9bd70718d702be577

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>