gcc: sync cflags/ldflags setup with Arch

For details see https://bugs.archlinux.org/task/71777
In theory this should speed up the builds, let's see..

also enable parallel install while at it, this was disabled 10 years
ago without any comment

mingw-w64-gcc/PKGBUILD

@@ -24,7 +24,7 @@ pkgver=13.1.0
 #_majorver=${pkgver:0:1}
 #_sourcedir=${_realname}-${_majorver}-${_snapshot}
 _sourcedir=${_realname}-${pkgver}
-pkgrel=1
+pkgrel=2
 pkgdesc="GCC for the MinGW-w64"
 arch=('any')
 mingw_arch=('mingw64' 'ucrt64')
@@ -258,11 +258,17 @@ build() {
     sed -i 's,^STRIP_FOR_TARGET=.*$,STRIP_FOR_TARGET=true,g' Makefile
   fi
 
-  make V=1 all
+  # https://gcc.gnu.org/onlinedocs/gccint/Makefile.html
+  # https://bugs.archlinux.org/task/71777
+  make -O STAGE1_CFLAGS="-O2" \
+          BOOT_CFLAGS="$CFLAGS" \
+          BOOT_LDFLAGS="$LDFLAGS" \
+          LDFLAGS_FOR_TARGET="$LDFLAGS" \
+          all
 
   rm -rf ${srcdir}${MINGW_PREFIX}
 
-  make -j1 DESTDIR=${srcdir} install
+  make DESTDIR=${srcdir} install
   if [ "$_enable_ada" == "yes" ]; then
     mv ${srcdir}${MINGW_PREFIX}/lib/gcc/${MINGW_CHOST}/${pkgver%%+*}/adalib/*.dll ${srcdir}${MINGW_PREFIX}/bin/
   fi