Harmonize variable names used to refer to guile modules in all makefiles

This makes the makefiles more similar and hence
easier to understand and maintain
Fix a few deprecation warnings when building and testing with guile 2
Guile module structure cleanup

- fix circular dependency between business-reports.scm and owner-report.scm
- split out gettext routines to properly handle N_ for all files that use it
- remove some unused module inclusions
- tax module depends on app-utils -> fix build order
Pre-compile scm files when building with guile 2

With pre-compiled scm files, guile will no longer
resort to autocompilation at first startup. This avoids
bugs like downstream
https://bugzilla.redhat.com/show_bug.cgi?id=1151870
(Where autocompiled files for an older version of
gnucash are more recent than the source files installed
with the newer version)

Note that a new variable GUILE_LOAD_COMPILED_PATH is added
for this in the environment file. This variable should probably
be updated in the OSX and Windows build scrips if/when these
are building with guile 2.

Strictly speaking this new variable is only needed for guile 2.
However since guile 1.8 simply ignores the environment variable,
it's just set unconditionally regardless of the guile version
being active.
Guile 2 pre-compilation improvements

1. Tell the engine to look in the build dir for the backend modules
   Guile compilation loads the engine libraries for
   some guile modules. This triggers the engine
   initialization code which tries to load the xml backend module
   and optionally the dbi module as well.
   By default it looks for these modules in the installed directory
   but during build they are not installed yet, so use a few
   environment variables to tell the engine to look in the build
   dir instead.

2. Wait for the c library to be linked before attempting to
   compile the guile files. Some guile files need the library
   to be available.

Note that neither fix is necessary for all directories that has
scheme files to compile. For consistency between makefiles, I have
chosen to add them to all, which won't hurt.
Use one common base directory to store all scheme files and modules

This makes it easier for humans to locate a file after
installation and simplifies the build system.
Omit extension on calls to load-extension

.so was platform specific. By omitting the extension
guile properly finds the right library on both linux
and Windows (haven't tested on OS X but I assume it
to work there as well).