separate handling of generated files
separate handling of generated files

One might assume that when a condition is false the statements inside
the `if` block "do not happen".  This is true (`am__gmetad_SOURCES`
will not have the riemann related files appended) but also wrong:
autotools looks at what might have been and append the files to
`am__gmetad_SOURCES_DIST` anyway.  Since some of these files are
generated this results in invalid make targets.

autotools documentation addresses the case of generated source files
at
http://www.gnu.org/savannah-checkouts/gnu/automake/manual/html_node/Sources.html

fix #164

ganglia/monitor-core,torkelsson/monitor-core,NoodlesNZ/monitor-core,phreakocious/monitor-core,ganglia/monitor-core,mjzhou/monitor-core,phreakocious/monitor-core,fastly/monitor-core,ganglia/monitor-core,fastly/monitor-core,ganglia/monitor-core,lawrencewu/monitor-core,lawrencewu/monitor-core,phreakocious/monitor-core,lawrencewu/monitor-core,lawrencewu/monitor-core,phreakocious/monitor-core,fastly/monitor-core,torkelsson/monitor-core,torkelsson/monitor-core,mjzhou/monitor-core,NoodlesNZ/monitor-core,torkelsson/monitor-core,torkelsson/monitor-core,mjzhou/monitor-core,ganglia/monitor-core,fastly/monitor-core,NoodlesNZ/monitor-core,fastly/monitor-core,lawrencewu/monitor-core,NoodlesNZ/monitor-core,lawrencewu/monitor-core,fastly/monitor-core,fastly/monitor-core,phreakocious/monitor-core,NoodlesNZ/monitor-core,phreakocious/monitor-core,mjzhou/monitor-core,ganglia/monitor-core,torkelsson/monitor-core,ganglia/monitor-core,mjzhou/monitor-core,NoodlesNZ/monitor-core,lawrencewu/monitor-core,mjzhou/monitor-core,NoodlesNZ/monitor-core,torkelsson/monitor-core,mjzhou/monitor-core,ganglia/monitor-core,phreakocious/monitor-core