Clarify message when failing with 'requires v4.7.4' error.

[WardF]

• Fixes Requires netCDF 4.7.4 or greater, but 4.7.4 is already installed #278

There may be an elegant solution, but for now we will just provide instructions alongside the error. When cmake invokes CHECK_LIBRARY_EXISTS() against a static library (such as libnetcdf.a), the dependencies must be specified manually, since there is no way to infer these dependencies with static libraries; this can be handled programmatically in the CMakeLists.txt file, where we check for nc-config and, if present, use the output of nc-config --libs, but for now we will just present this information when encountering this error.

The fix is to set LDFLAGS to the output from nc-config --libs.