A number of changes in this commit:
1. Variable namespace cleanup:
Rename non-configurable variables as MIQ_XYZ.
2. Internal target namespace cleanup
Rename intermediate targets to begin with a dot (.)
For example .build, .prebuild, etc.
This impacts makefiles that hook for example to prebuild
(now they need to hook to the .prebuild target)
3. Target, variant and build-environment variables names
For example, dependencies on $(TARGET) moved from DEFINES_xyz
to DEFINES_TARGET_xyz instead of DEFINES_xyz, and same thing
for includes, sources, etc.
4. Protection of special user-level top-level targets
A target like log-% no longer interferes with a file name such
as log-file.c
5. Better support for multiple products
When building foo.exe and bar.lib, there can be different
sources for each project, i.e. $(SOURCES_foo) and $(SOURCES_bar).
Christophe de Dinechin