README: add note on RHEL/CentOS 6

Add note on autoconf >2.69 being required for compilation under
RHEL/CentOS.

Closes #178.
README.md: fix anchor tags for CentOS/RHEL section

MAke the use of anchor tags consistent for the change introduced in
5cd3cbe.
Merge pull request #183 from rjw57/issue-178

README: add note on RHEL/CentOS 6
Use CMAKE_C_FLAGS variable to set compiler flags.

If the compiler is some GNU-alike variant, set the compiler flags to use
the gnu99 dialect of C and enable all warnings.

Non-GNU compilers may have to have their own magic added to set dialect
and enable warnings.

Closes #179.
Merge pull request #187 from rjw57/issue-179

Use CMAKE_C_FLAGS variable to set compiler flags.