app, meson: disable GimpBacktrace on Linux without execinfo.h.

If execinfo.h was not found, do not try to compile gimpbacktrace-linux.c
because the backtrace() API is not optional there.

Also I'm further improving the meson summary, regarding "Dashboard
Backtraces". Rather than just yes/no to say if the traces are detailed,
let's go for finer-grained state, saying if the traces are completely
deactivated ("no", e.g. when execinfo.h is not found), or "rough" (when
libunwind and libbacktrace are not found) or "partially detailed" (one
of these is not found) and finally "detailed".
Also add info on missing dependency between parentheses to help
packagers find the proper dependencies to get to the "detailed" state.

app/core/gimpbacktrace-backend.h

@@ -22,7 +22,7 @@
 #define __GIMP_BACKTRACE_BACKEND_H__
 
 
-#ifdef __gnu_linux__
+#if defined (__gnu_linux__) && defined (HAVE_EXECINFO_H)
 # define GIMP_BACKTRACE_BACKEND_LINUX
 #elif defined (G_OS_WIN32) && defined (ARCH_X86)
 # define GIMP_BACKTRACE_BACKEND_WINDOWS

meson.build

@@ -601,6 +601,36 @@ libunwind = ( get_option('libunwind')
 )
 conf.set('HAVE_LIBUNWIND', libunwind.found())
 
+## Check for backtrace() API
+# In musl, backtrace() is in the libexecinfo library.
+# In glibc, it is internal (there we only need the header).
+# So we look for both cases, so that we are able to link to libexecinfo
+# if it exists. Cf. !455.
+opt_execinfo = cc.find_library('execinfo', has_headers: ['execinfo.h'], required: false)
+have_execinfo_h = opt_execinfo.found() or cc.has_header('execinfo.h')
+conf.set('HAVE_EXECINFO_H', have_execinfo_h ? 1 : false)
+
+# See app/core/gimpbacktrace-backend.h for supported platforms.
+dashboard_backtrace='no (unsupported platform)'
+if platform_windows
+  if have_x86
+    dashboard_backtrace='yes'
+  else
+    dashboard_backtrace='no (x86 only on Windows)'
+  endif
+elif platform_linux
+  if not have_execinfo_h
+    dashboard_backtrace='no (missing: execinfo.h)'
+  elif not libbacktrace.found() and not libunwind.found()
+    dashboard_backtrace='rough (missing: libbacktrace and libunwind)'
+  elif not libbacktrace.found()
+    dashboard_backtrace='partially detailed (missing: libbacktrace)'
+  elif not libunwind.found()
+    dashboard_backtrace='partially detailed (missing: libunwind)'
+  else
+    dashboard_backtrace='detailed'
+  endif
+endif
 
 ## Check for Dr. Mingw
 drmingw = no_dep
@@ -613,11 +643,6 @@ if platform_windows
 endif
 conf.set('HAVE_EXCHNDL', drmingw.found())
 
-# See app/core/gimpbacktrace-backend.h for supported platforms.
-detailed_backtraces = (
-  (platform_linux and (libbacktrace.found() or libunwind.found())) or
-  (platform_windows and have_x86)
-)
 
 ################################################################################
 # Check for x11 support
@@ -1451,13 +1476,6 @@ foreach header : [
   conf.set(header['m'], cc.has_header(header['v']) ? 1 : false)
 endforeach
 
-# In musl, backtrace() is in the libexecinfo library.
-# In glibc, it is internal (there we only need the header).
-# So we look for both cases, so that we are able to link to libexecinfo
-# if it exists. Cf. !455.
-opt_execinfo = cc.find_library('execinfo', has_headers: ['execinfo.h'], required: false)
-conf.set('HAVE_EXECINFO_H', opt_execinfo.found() or cc.has_header('execinfo.h') ? 1 : false)
-
 
 ################################################################################
 # Check for shared memory handling
@@ -2097,7 +2115,7 @@ final_message = [
 '''  Relocatable Bundle:        @0@'''.format(relocatable_bundle),
 '''  Default ICC directory:     @0@'''.format(icc_directory),
 '''  32-bit DLL folder (Win32): @0@'''.format(get_option('win32-32bits-dll-folder')),
-'''  Detailed backtraces (Dashboard): @0@'''.format(detailed_backtraces),
+'''  Dashboard backtraces:      @0@'''.format(dashboard_backtrace),
 '''  Binary symlinks:           @0@'''.format(enable_default_bin),
 '''  OpenMP:                    @0@'''.format(have_openmp),
 '',