Handle the case where h5cc -show returns multiple lines

[wangvsa]

Description

Fix issue #793.
When determining the HDF5 C compiler, only examine the first line. Otherwise, the generated Makefile might be incorrect.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Testing (addition of new tests or update to current tests)
• Documentation (a change to man pages or other documentation)