Fix compilation on Xcode 4.6

Removes the inline declaration for the clang compiler, which fixes the following compilation error in lmms:
```
Linking C shared module imp_1199.so
Undefined symbols for architecture x86_64:
   "_impulse2freq", referenced from:
      _mk_imps in imp_1199.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]: *** [plugins/LadspaEffect/swh/imp_1199.so] Error 1
make[1]: *** [plugins/LadspaEffect/swh/CMakeFiles/imp_1199.dir/all] Error 2
```

Note, this didn't seem to be a problem with Clang 5.x, but we're trying to add compat for older OSs, hence the old Xcode version. :)