Disable ccache by default

Disable ccache by default

There are too many use cases where build trees might be used on
different nodes where ccache might not be available, or with compilers
that it cannot support to continue to enable it by default. If the
user opts in, then the build system searches for ccache and gives a
fatal error if it is not found, or if the compiler is not supported.

The wrappers used by clang-tidy and clang-analyzer do not work
with ccache, so those configurations issue a fatal error if
ccache is enabled.

Made various minor aspects conform to style for naming cache
variables, use of endif(), etc.

Refs #2688

Change-Id: I55c8d4a8a07ada704c49d4f99c1557a7ab97f353