Check if external compression libraries are available at run-time.

This patch ensures that Tor checks if our compression backend libraries
are actually available at run-time. This problem happens if Tor is
compiled with weak linking where the lack of the external library
provided functions will lead to a NULL dereference first time they are
called.

See: https://bugs.torproject.org/22926
1 job for bugs/22926 in 4 minutes and 56 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #51206089
build

00:04:56

 
Name Stage Failure
failed
build Test
                                   ^
src/common/compat.h:199:51: note: in definition of macro 'PREDICT_UNLIKELY'
#define PREDICT_UNLIKELY(exp) __builtin_expect(!!(exp), 0)
^
cc1: all warnings being treated as errors
make: *** [src/common/compress_lzma.o] Error 1
Makefile:5739: recipe for target 'src/common/compress_lzma.o' failed
cat: test-suite.log: No such file or directory
ERROR: Job failed: exit code 1