package/pkg-cmake.mk: add note about BUILD_STATIC_LIBS

package/pkg-cmake.mk: add note about BUILD_STATIC_LIBS

As BUILD_STATIC_LIBS is not a standard cmake variable (while
BUILD_SHARED_LIBS is) we shouldn't add it in pkg-cmake.mk, although
for some packages that would make sense. Therefore, add a note so we
don't forget about this abnormality.

See: https://cmake.org/cmake/help/v3.8/manual/cmake-variables.7.html#variables-that-change-behavior

Signed-off-by: Jan Heylen <jan.heylen@nokia.com>
[Thomas: rework the comment in the code.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>