cmake: always depend on threads, not only in the xmp case

[bkuhls]

Maybe fixes #3

[clanmills]

Thanks for this. I'm intending to work on #3 during the weekend. I'll accept your patch. I'll be having another sniff in #3 tomorrow. Thank You very much for this contribution.

Incidentally, I'm getting a build error on MacOS-X with your cmake/gettext patch. I normally use -DEXIV2_ENABLE_NLS=0 to keep away from trouble with gettext on the Mac. I'll take ownership and resolve this.

[ 51%] Linking CXX shared library libexiv2.dylib
Undefined symbols for architecture x86_64:
  "_libintl_bindtextdomain", referenced from:
      bindtextdomain(char const*, char const*) in types.cpp.o
  "_libintl_dgettext", referenced from:
      dgettext(char const*, char const*) in types.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [src/libexiv2.26.0.0.dylib] Error 1
make[1]: *** [src/CMakeFiles/exiv2lib.dir/all] Error 2
make: *** [all] Error 2
520 rmills@rmillsmbp:~/gnu/git/exiv2/build $ 

[clanmills]

I'm getting a mysterious and similar link error on Cygwin: Linux is OK.

[ 51%] Linking CXX shared library ../bin/cygexiv2-26.dll
CMakeFiles/exiv2lib.dir/types.cpp.o:types.cpp:(.text+0x1ccf): undefined reference to `libintl_bindtextdomain'
CMakeFiles/exiv2lib.dir/types.cpp.o:types.cpp:(.text+0x1ccf): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `libintl_bindtextdomain'
CMakeFiles/exiv2lib.dir/types.cpp.o:types.cpp:(.text+0x1ce6): undefined reference to `libintl_dgettext'
CMakeFiles/exiv2lib.dir/types.cpp.o:types.cpp:(.text+0x1ce6): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `libintl_dgettext'
collect2: error: ld returned 1 exit status
make[2]: *** [src/CMakeFiles/exiv2lib.dir/build.make:1583: bin/cygexiv2-26.dll] Error 1
make[1]: *** [CMakeFiles/Makefile2:349: src/CMakeFiles/exiv2lib.dir/all] Error 2
make: *** [Makefile:128: all] Error 2
533 rmills@rmillsmbp-w7:~/gnu/git/exiv2/build $