Дрова от модема скачай с сайта производителя.

root # emerge -vat dev-util/cmake

These are the packages that would be merged, in reverse order:

Calculating dependencies... done!

[nomerge       ] dev-util/cmake-3.2.1::gentoo  USE="ncurses qt5 -doc -emacs -qt4 {-test}" 

[ebuild  N     ]  dev-libs/jsoncpp-0.10.1::gentoo  USE="-doc {-test}" 0 KiB

[ebuild  N     ]   dev-util/cmake-3.2.1::gentoo  USE="ncurses qt5 -doc -emacs -qt4 {-test}" 0 KiB

Total: 2 packages (2 new), Size of downloads: 0 KiB

 * Error: circular dependencies:

(dev-util/cmake-3.2.1:0/0::gentoo, ebuild scheduled for merge) depends on

 (dev-libs/jsoncpp-0.10.1:0/0::gentoo, ebuild scheduled for merge) (buildtime)

  (dev-util/cmake-3.2.1:0/0::gentoo, ebuild scheduled for merge) (buildtime)

  * Note that circular dependencies can often be avoided by temporarily

  * disabling USE flags that trigger optional dependencies.

и jsoncpp без cmake не установить.

в ебилде убираем зависимость от jsoncpp и 

        >=dev-libs/jsoncpp-0.6.0_rc2

втравляем  в src_configure() {

-DCMAKE_USE_SYSTEM_LIBRARY_JSONCPP=OFF

сам ебилд

root # emerge cmake jsoncpp