Fix build with -Dnls=false on toolchain providing libintl.h

[ffontaine]

Always check for libintl.h to avoid a clash with ncmpc fake gettext. If libintl.h is available, include it and undef getext before defining fake gettext. Indeed, a toolchain can provide an external gettext definition in libintl.h which will be included indirectly into ncmpc (e.g., though include <chrono>) resulting in the following build failure if the user disables NLS through -Dnls=false:

In file included from ../src/save_playlist.cxx:8:
/home/buildroot/autobuild/instance-2/output-1/host/or1k-buildroot-linux-gnu/sysroot/usr/include/libintl.h:39:14: error: expected unqualified-id before 'const'
   39 | extern char *gettext (const char *__msgid)
      |              ^~~~~~~
/home/buildroot/autobuild/instance-2/output-1/host/or1k-buildroot-linux-gnu/sysroot/usr/include/libintl.h:39:14: error: expected ')' before 'const'
../src/i18n.h:22:20: note: to match this '('
   22 | #define gettext(x) (x)
      |                    ^

Fixes:

• http://autobuild.buildroot.org/results/c860e64909dd3b7fa033db0970e1cfc1376c4ec5
• http://autobuild.buildroot.org/results/36eab556fac80124755bff4110af3e43fe6edd78