C/C++ is provided with z/OS, as is the run time. I'm not as comfortable with the stability of the C++ runtime; IBM appears to keep changing the feature set, but the C runtime should be extremely stable.
FWIW, your z/OS development environment is ancient history. The current z/OS is 1.11, with 1.12 shipping in September. You really should upgrade.
The other mainframe issue is there are really 2, possibly 3, environments you should think about.
- Regular batch / TSO. This environment is EBCDIC character set based, and the I/O runtime is mapped, generally pretty will, into z/OS data set structures.
- UNIX System Services environment. This environment is also EBCDIC, but it has a file system very much like the UNIX environment you're probably familiar with.
- Mainframe LINUX. This is an ASCII environment, but it sits off in its little corner with very limited access to z/OS data, both UNIX System Services data as well as regular z/OS data. This means very limited access to z/OS DB2 or IMS type data, and you can't use the extremely effective z/OS tools like sort.