Add Linux support for external tirpc library

[jlmuir]

Add support on Linux for using a tirpc library that is not a system library.

[AppVeyorBot]

❌ Build asyn 1.0.290 failed (commit 8cacd75dde by @jlmuir)

[jwlodek]

I think in most cases in AD for example, ***_EXTERNAL specifies whether to use a version of the library built as a part of the EPICS build system or not, rather than a system version vs. source install. Maybe we could automatically use the system version if TIRPC_INCLUDE and TIRPC_LIB are not set, and we can comment them out by default? Then we wouldn't need the TIRPC_EXTERNAL.

Something like: https://github.com/areaDetector/ADCore/blob/0ebde8129e652876b7fd71b459275725f70b2da6/ADApp/commonDriverMakefile#L230

[jlmuir]

I was basically trying to be consistent with areaDetector's configure/EXAMPLE_CONFIG_SITE.local, which contains the following comment:

# Configure which 3rd party libraries to use and where to find them.
# For each library XXX the following definitions are used:
# WITH_XXX      Build the plugins and drivers that require this library.
#               Build the source code for this library in ADSupport if XXX_EXTERNAL=NO.
# XXX_EXTERNAL  If NO then build the source code for this library in ADSupport.
# XXX_INCLUDE   If XXX_EXTERNAL=YES then this is the path to the include files for XXX.
#               However, if XXX is a system library whose include files are in a 
#               standard include search path then do not define XXX_INCLUDE.
# XXX_LIB       If XXX_EXTERNAL=YES then this is the path to the library files for XXX.
#               However, if XXX is a system library whose library files in a 
#               standard library search path then do not define XXX_LIB.

But I'd be happy with the approach you proposed too. Thanks!