CMakeLists.txt: libcgi is in C
CMakeLists.txt: libcgi is in C

By default, CMake checks that both C and C++ compilers are
available. However, since libcgi is only C, there's no need for a C++
compiler check. Therefore, this commit adjusts the project() variable
definition to only require C language support.

Signed-off-by: Thomas Petazzoni <a5d87d2bd7c424ee5cbdb4e978635bf653d5a005@free-electrons.com>

rafaelsteil/libcgi,rafaelsteil/libcgi,LeSpocky/libcgi,LeSpocky/libcgi