build: use per-target '_CPPFLAGS' for those currently using default

Automake documents that doing this will make it choose a different name
for intermediate object files even when sharing source files across
targets of same Makefile.am.

Up to automake 1.13.1 target's intermediate object files were placed
in the build subdirectory of the target. We depended on this, probably
undocumented behavior, to achieve same behavior as if a per-target flag
had been specified when building targets that actually belong to
different Makefile.am files.

It seems automake 1.13.2 is going to break behavior mentioned above.

So, lets use a documented behavior in order to achieve same purpose,
across automake versions, no matter where automake wishes to place
intermediate object files.

Our build targets that already were using a per-target '_CFLAGS' or
'_CPPFLAGS' need no 'fixing', these were already 'fixed'. The only
Makefile.am or Makefile.in files in libcurl's source tree touched by
this 'fix' are tests/libtest/Makefile.inc and tests/unit/Makefile.inc.