support/configure.inc: fix crosscompiling

--cc= can be whatever. there should be no check if generated
binaries can run while crosscompiling.
Power off LNB when idle

Try to reduce power, when no streams are captured from DVB-S/S2 devices.
Fix the adapter_mask handling for linuxdvb
build: check for qsort_r independent of platform

Rather than check for specific glibc version on PLATFORM_LINUX, check
the existance of qsort_r.

This fixes the following compile error with musl libc:

CC              src/main.o
In file included from src/main.c:39:0:
src/tvheadend.h:608:20: error: missing binary operator before token "("
 #if !__GLIBC_PREREQ(2,8)
                    ^
build: check for gcc version independent of platform

Use the gcc specific __GNUC__ and __GNUC__ minor to determine gcc version
rather than using the GNU libc specific __GNUC_PREREQ.

This fixes the following compile error with musl libc:
src/descrambler/capmt.c:79:18: error: missing binary operator before token "("
 #if __GNUC_PREREQ(4, 3)
                  ^
build: detect wordsize in a portable way

There are no standard that says that __WORDSIZE should be defined or
in what include file. So in case its not defined we fallback to a
POSIX compliant way to determine wordsize.

This fixes build with 64 bit musl libc.

See https://tvheadend.org/issues/1983
build: typecast NGREG to int

Fixes the following error with musl libc:

src/trap.c: In function 'traphandler':
src/trap.c:178:3: error: format '%d' expects argument of type 'int', but argument 4 has type 'long unsigned int' [-Werror=format=]
   snprintf(tmpbuf, sizeof(tmpbuf), "Register dump [%d]: ", NGREG);
      ^
Merge remote-tracking branch 'origin/pr/330'

Fixes #1983
Merge remote-tracking branch 'origin/pr/328'
Merge remote-tracking branch 'origin/pr/327'
Merge remote-tracking branch 'origin/pr/326'