configure: use a platform-neutral prefix

Now that the installation is relocatable, there is no need to compile a
Windows-format prefix into Win32 binaries.  Instead, the prefix will
only be used to compute installation-relative paths, and it can be
any string.

Drop the "Program Files" path completely: it is only usable on English
versions of Windows; therefore, using the NSIS installer to get the
"correct" path to the Program Files folder is recommended, and NSIS
works just as well with any prefix.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -969,7 +969,7 @@ if test "$mingw32" = "yes" ; then
   # MinGW needs -mthreads for TLS and macro _MT.
   CFLAGS="-mthreads $CFLAGS"
   write_c_skeleton;
-  prefix="c:/Program Files/QEMU"
+  prefix="/qemu"
   qemu_suffix=""
   libs_qga="-lws2_32 -lwinmm -lpowrprof -lwtsapi32 -lwininet -liphlpapi -lnetapi32 $libs_qga"
 fi
@@ -7795,17 +7795,9 @@ echo "strip = $(meson_quote $strip)" >> $cross
 echo "windres = $(meson_quote $windres)" >> $cross
 if test -n "$cross_prefix"; then
     cross_arg="--cross-file config-meson.cross"
-    # Hack: Meson expects an absolute path for the *build* machine
-    # for the prefix, so add a slash in front of a Windows path that
-    # includes a drive letter.
-    #
-    # See https://github.com/mesonbuild/meson/issues/7577.
     echo "[host_machine]" >> $cross
     if test "$mingw32" = "yes" ; then
         echo "system = 'windows'" >> $cross
-        case $prefix in
-            ?:*) pre_prefix=/ ;;
-        esac
     fi
     if test "$linux" = "yes" ; then
         echo "system = 'linux'" >> $cross
@@ -7834,16 +7826,16 @@ mv $cross config-meson.cross
 
 rm -rf meson-private meson-info meson-logs
 NINJA=${ninja:-$PWD/ninjatool} $meson setup \
-        --prefix "${pre_prefix}$prefix" \
-        --libdir "${pre_prefix}$libdir" \
-        --libexecdir "${pre_prefix}$libexecdir" \
-        --bindir "${pre_prefix}$bindir" \
-        --includedir "${pre_prefix}$includedir" \
-        --datadir "${pre_prefix}$datadir" \
-        --mandir "${pre_prefix}$mandir" \
-        --sysconfdir "${pre_prefix}$sysconfdir" \
-        --localstatedir "${pre_prefix}$local_statedir" \
-        -Ddocdir="${pre_prefix}$docdir" \
+        --prefix "$prefix" \
+        --libdir "$libdir" \
+        --libexecdir "$libexecdir" \
+        --bindir "$bindir" \
+        --includedir "$includedir" \
+        --datadir "$datadir" \
+        --mandir "$mandir" \
+        --sysconfdir "$sysconfdir" \
+        --localstatedir "$local_statedir" \
+        -Ddocdir="$docdir" \
         -Dqemu_suffix="$qemu_suffix" \
         -Doptimization=$(if test "$debug" = yes; then echo 0; else echo 2; fi) \
         -Ddebug=$(if test "$debug_info" = yes; then echo true; else echo false; fi) \