Modify Unix ./configure script stat invocation to support busybox-style systems.

`./configure` will now formally probe which `stat` parameter to use, and in addition to `--format=` (GNU) and `-f` (BSD) also try `-c` (busybox). Should fix issue #400.

Author: c-lipka

changes.txt

@@ -130,6 +130,7 @@ Reported via GitHub:
     -lboost_system with Boost 1.66)
   - #342 ("AX_FIX_INCORRECT_PATH is broken", configure script failing to
     properly handle `.` directory in `C_INCLUDE_PATH` or `CPLUS_INCLUDE_PATH`)
+  - #400 ("Building on postmarketOS on PinePhone")
   - #403 ("povray 3.7.0.8 does not build on macOS 11.0 Big Sur").
 
 Reported via the Newsgroups:

unix/configure.ac

@@ -217,11 +217,16 @@ echo "trying stat --format='%u,%g' ." >>config.log
 if stat --format='%u,%g' . >>config.log 2>>config.log && test x`stat --format=Foo .` = x"Foo"; then
   stat_format="--format="
 else
-  echo "trying stat --f '%u,%g' ." >>config.log
+  echo "trying stat -f '%u,%g' ." >>config.log
   if stat -f '%u,%g' . >>config.log 2>>config.log && test x`stat -f Foo .` = x"Foo"; then
     stat_format="-f "
   else
-    AC_MSG_FAILURE([stat command does not behave as required])
+    echo "trying stat -c'%u,%g' ." >>config.log
+    if stat -c '%u,%g' . >>config.log 2>>config.log && test x`stat -c Foo .` = x"Foo"; then
+      stat_format="-c "
+    else
+      AC_MSG_FAILURE([stat command does not behave as required])
+    fi
   fi
 fi
 AC_MSG_RESULT([$stat_format])