uppercase with-flags: --with-foo-CFLAGS/--with-bar-LIBS

rather than --with-foo-cflags/--with-bar-libs
this should make the meaning more clear.

also added code to support the legacy behaviour

Closes https://github.com/umlaeute/Gem/issues/36
use "subdir-objects" for videoVIDS
allow to disable a framework explicitely

also replace the buildflags GEM_FRAMEWORK_foo with GEM_FRAMEWORK_foo_LIBS (and
introduce GEM_FRAMEWORK_foo_CFLAGS)

Closes https://github.com/umlaeute/Gem/issues/66

the code for manually settings framework-FLAGS is not working yet
use GEM_FRAMEWORK_foo_{CFLAGS|LIBS} rather than GEM_FRAMEWORK_foo
only build {film|video}Darwin if both QuickTime and Carbon is present
move am_conditionals outside the "-framework" test

so they can also be set when the user manually sets build-flags
allow to force framework build-flags

Closes https://github.com/umlaeute/Gem/issues/66
get rid of GEM_FRAMEWORK_foo flag

it's been replaced by GEM_FRAMEWORK_foo_LIBS (resp. ..._CFLAGS)