[MinGW] Clear prefix and move the directory to DESTDIR

Also the destination will be created at the installation.

.github/workflows/mingw.yml

@@ -74,7 +74,6 @@ jobs:
           }
       - name: misc setup, autoreconf
         run: |
-          mkdir install
           mkdir temp
           cd src
           sh -c "autoreconf -fi"
@@ -88,7 +87,7 @@ jobs:
           [Console]::InputEncoding  = [System.Text.Encoding]::GetEncoding("IBM437")
           $config_args = "--build=$env:CHOST --host=$env:CHOST --target=$env:CHOST"
           Write-Host $config_args
-          sh -c "../src/configure --disable-install-doc --prefix=/install $config_args"
+          sh -c "../src/configure --disable-install-doc --prefix=/. $config_args"
           # Write-Host "-------------------------------------- config.log"
           # Get-Content ./config.log | foreach {Write-Output $_}
 
@@ -113,7 +112,7 @@ jobs:
           $PSDefaultParameterValues['*:Encoding'] = 'utf8'
           [Console]::OutputEncoding = [System.Text.Encoding]::GetEncoding("IBM437")
           [Console]::InputEncoding  = [System.Text.Encoding]::GetEncoding("IBM437")
-          make DESTDIR=.. install-nodoc
+          make DESTDIR=../install install-nodoc
 
       - name: test
         timeout-minutes: 5