Does not compile on Alpine (musl)

[ventosus]

ninja: job failed: c++ -Insmd.p -I. -I.. -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -O0 -g -pthread -MD -MQ nsmd.p/src_nsmd.cpp.o -MF nsmd.p/src_nsmd.cpp.o.d -o nsmd.p/src_nsmd.cpp.o -c ../src/nsmd.cpp
../src/nsmd.cpp:47:10: fatal error: fts.h: No such file or directory
   47 | #include <fts.h>
      |          ^~~~~~~
compilation terminated.

musl does not include 'fts_*' functions, they are in a separate library 'musl-fts' which needs to be linked to:

I've not found out how to detect musl in meson, but looking for 'fts_open' seems to be the better check.

Possible fix to get inspired from:

diff --git a/meson.build b/meson.build
index ff5364d..92c1c0b 100644
--- a/meson.build
+++ b/meson.build
@@ -41,6 +41,8 @@ fltkdep = cc.find_library('fltk', required: get_option('nsm-legacy-gui') or get_
 fltkimagesdep = cc.find_library('fltk_images', required: get_option('nsm-legacy-gui'))
 fluid = find_program('fluid', required: get_option('nsm-proxy'))
 
+has_fts = cc.has_function('fts_open')
+ftsdep = dependency('musl-fts', required: not has_fts)
 
 ##############
 #Build Targets
@@ -48,7 +50,7 @@ fluid = find_program('fluid', required: get_option('nsm-proxy'))
 
 executable('nsmd',
     sources: ['src/nsmd.cpp', 'src/debug.cpp', 'src/Endpoint.cpp', 'src/file.cpp', 'src/Thread.cpp'],
-    dependencies: [liblodep, threaddep],
+    dependencies: [liblodep, threaddep, ftsdep],
     install: true,
     )