With the module search path cleverly handled with different install.h
configurations, the genstatic script inserted absolute paths into the
generated clib.c file. This didn't fail on Windows CI as this is an
in-source build. For out-of-source builds, it's crucial that clib.c can
refer to both .c files in the source directory and those generated in
the build directory. As a fix, the genstatic invocation now uses the -I
flag.
This patch also improves the handling of include paths to find the .c.
files mentioned above by trimming down the scope of this property to
clib.c only. Also, there is no need to manually tell the preprocessor
where to look for generated .c, as they live relative to clib.c anyhow.
