meson: generalise build_apple to has_dlvsym

build_apple was introduced in 756dca as a proxy for the fact that Apple's libc
doesn't have dlvsym(), which is glibc-specific so also isn't present in other
libc implementations such as musl.

Instead of detecting whether we are building for Apple or not, just probe the to
see if we have dlvsym.

meson.build

@@ -81,15 +81,12 @@ endif
 # The remaining platform specific API for GL/GLES are enabled
 # depending on the platform we're building for
 if host_system == 'windows'
-  build_apple = false
   build_wgl = true
   has_znow = true
 elif host_system == 'darwin'
-  build_apple = true
   build_wgl = false
   has_znow = false
 else
-  build_apple = false
   build_wgl = false
   has_znow = true
 endif

test/meson.build

@@ -1,3 +1,6 @@
+dl_dep = cc.find_library('dl', required: false)
+has_dlvsym = cc.has_function('dlvsym', dependencies: dl_dep)
+
 has_gles1 = gles1_dep.found()
 has_gles2 = gles2_dep.found()
 build_x11_tests = enable_x11 and x11_dep.found()
@@ -92,8 +95,8 @@ if build_glx
     [ 'glx_has_extension_nocontext', [ 'glx_has_extension_nocontext.c' ], [], [], true ],
     [ 'glx_static', [ 'glx_static.c' ], [ '-DNEEDS_TO_BE_STATIC'], [ '-static' ], libtype == 'static' ],
     [ 'glx_shared_znow', [ 'glx_static.c', ], [], [ '-Wl,-z,now' ], has_znow ],
-    [ 'glx_alias_prefer_same_name', [ 'glx_alias_prefer_same_name.c', 'dlwrap.c', 'dlwrap.h' ], [], [ '-rdynamic' ], not build_apple ],
+    [ 'glx_alias_prefer_same_name', [ 'glx_alias_prefer_same_name.c', 'dlwrap.c', 'dlwrap.h' ], [], [ '-rdynamic' ], has_dlvsym ],
-    [ 'glx_gles2', [ 'glx_gles2.c', 'dlwrap.c', 'dlwrap.h' ], [], [ '-rdynamic' ], not build_apple ],
+    [ 'glx_gles2', [ 'glx_gles2.c', 'dlwrap.c', 'dlwrap.h' ], [], [ '-rdynamic' ], has_dlvsym ],
   ]
 
   foreach test: glx_tests
@@ -114,7 +117,7 @@ if build_glx
     endif
   endforeach
 
-  if not build_apple
+  if has_dlvsym
     # GLX/EGL tests
     if build_egl
       glx_egl_sources = [