Allow syncing the source to the pinned version by setting SYNC=1 when running the build scripts

This should allow updating an existing build to the desired, pinned
version.

build-compiler-rt.sh

@@ -15,9 +15,7 @@ export PATH=$PREFIX/bin:$PATH
 
 if [ ! -d compiler-rt ]; then
     git clone -b master https://github.com/llvm-mirror/compiler-rt.git
-    cd compiler-rt
-    git checkout 1d871d6cd3fed01cd50dd63e743bd2ea6e65eab6
-    cd ..
+    CHECKOUT=1
 fi
 
 # Add a symlink for i386 -> i686; we normally name the toolchain
@@ -26,6 +24,12 @@ fi
 ln -sf i686-w64-mingw32 $PREFIX/i386-w64-mingw32
 
 cd compiler-rt
+
+if [ -n "$SYNC" ] || [ -n "$CHECKOUT" ]; then
+    [ -z "$SYNC" ] || git fetch
+    git checkout 1d871d6cd3fed01cd50dd63e743bd2ea6e65eab6
+fi
+
 for arch in $ARCHS; do
     buildarchname=$arch
     libarchname=$arch

build-libcxx.sh

@@ -14,27 +14,25 @@ export PATH=$PREFIX/bin:$PATH
 
 if [ ! -d libunwind ]; then
     git clone -b master https://github.com/llvm-mirror/libunwind.git
-    cd libunwind
-    git checkout 2ddcf2461daa5d61c543474aed06b12a8b9ad816
-    cd ..
+    CHECKOUT_LIBUNWIND=1
 fi
 if [ ! -d libcxxabi ]; then
     git clone -b master https://github.com/llvm-mirror/libcxxabi.git
-    cd libcxxabi
-    git checkout 05ba3281482304ae8de31123a594972a495da06d
-    cd ..
+    CHECKOUT_LIBCXXABI=1
 fi
 if [ ! -d libcxx ]; then
     git clone -b master https://github.com/llvm-mirror/libcxx.git
-    cd libcxx
-    git checkout 16eb426933cdb1c54fd6fbba5fc06cc5a4ca52b3
-    cd ..
+    CHECKOUT_LIBCXX=1
 fi
 
 LIBCXX=$(pwd)/libcxx
 MERGE_ARCHIVES=$(pwd)/merge-archives.sh
 
 cd libunwind
+if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBUNWIND" ]; then
+    [ -z "$SYNC" ] || git fetch
+    git checkout 2ddcf2461daa5d61c543474aed06b12a8b9ad816
+fi
 for arch in $ARCHS; do
     mkdir -p build-$arch
     cd build-$arch
@@ -69,6 +67,10 @@ done
 cd ..
 
 cd libcxxabi
+if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBCXXABI" ]; then
+    [ -z "$SYNC" ] || git fetch
+    git checkout 05ba3281482304ae8de31123a594972a495da06d
+fi
 for arch in $ARCHS; do
     mkdir -p build-$arch
     cd build-$arch
@@ -99,6 +101,10 @@ done
 cd ..
 
 cd libcxx
+if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBCXX" ]; then
+    [ -z "$SYNC" ] || git fetch
+    git checkout 16eb426933cdb1c54fd6fbba5fc06cc5a4ca52b3
+fi
 for arch in $ARCHS; do
     mkdir -p build-$arch
     cd build-$arch

build-llvm.sh

@@ -22,16 +22,27 @@ if [ ! -d llvm ]; then
     git svn init https://llvm.org/svn/llvm-project/llvm/trunk
     git config svn-remote.svn.fetch :refs/remotes/origin/master
     git svn rebase -l
-    git checkout c8f103e52b297f3e3e0ed1756e11373c68af3566
     cd tools/clang
     git svn init https://llvm.org/svn/llvm-project/cfe/trunk
     git config svn-remote.svn.fetch :refs/remotes/origin/master
     git svn rebase -l
-    git checkout fdd60aee9c0ea40fa0423d6e43821f53d07961d5
     cd ../lld
     git svn init https://llvm.org/svn/llvm-project/lld/trunk
     git config svn-remote.svn.fetch :refs/remotes/origin/master
     git svn rebase -l
+    cd ../../..
+    CHECKOUT=1
+fi
+
+if [ -n "$SYNC" ] || [ -n "$CHECKOUT" ]; then
+    cd llvm
+    [ -z "$SYNC" ] || git fetch
+    git checkout c8f103e52b297f3e3e0ed1756e11373c68af3566
+    cd tools/clang
+    [ -z "$SYNC" ] || git fetch
+    git checkout fdd60aee9c0ea40fa0423d6e43821f53d07961d5
+    cd ../lld
+    [ -z "$SYNC" ] || git fetch
     git checkout f4208caae12f685c06657aa6e2b9b1eda4adcdb4
     cd ../../..
 fi

build-mingw-w64.sh

@@ -14,13 +14,16 @@ export PATH=$PREFIX/bin:$PATH
 
 if [ ! -d mingw-w64 ]; then
     git clone git://git.code.sf.net/p/mingw-w64/mingw-w64
-    cd mingw-w64
-    git checkout 66fab9591c250ade399e9fe91ceda239a735649c
-    cd ..
+    CHECKOUT=1
 fi
 
 cd mingw-w64
 
+if [ -n "$SYNC" ] || [ -n "$CHECKOUT" ]; then
+    [ -z "$SYNC" ] || git fetch
+    git checkout 66fab9591c250ade399e9fe91ceda239a735649c
+fi
+
 if [ -z "$NOHEADERS" ]; then
     # TODO: Only install if they have changed? Otherwise this forces the CRT
     # to be rebuilt even if headers haven't changed. Currently set $NOHEADERS