Fixed linking with libecap after PKG_CHECK_MODULES changes.

Fixed linking with libecap after PKG_CHECK_MODULES changes.

Previous commit (r11126) made sense and seemed to work fine until I
discovered that touching a source file in src/adaptation/ecap and
running "make" there leads to libtool's "file not found" errors for some
libtool-generated internal file. However, "make clean all" would still
work.

To better diagnose the issue, I renamed PKG_CHECK_MODULES prefix to
EXTLIBECAP and our ecap/libecap to ecap/libxecap. The difference in
names allowed to distinguish external libecap-related names/failures
from internal ones, but it did not solve the problem.

Moving EXTLIBECAP_LIBS from adaptation/ecap/ to adaptation/ Makefile.am
helped.  Older code included external libecap in adaptation/Makefile.am
as well, perhaps to avoid similar problems. It does not make sense to
put external libecap there instead of the ecap-specific directory, but
it works.