Avoid GNU linker LTO warnings in Github workflows

Stop linker emitting warnings such as

  lto-wrapper: warning: using serial compilation of n LTRANS jobs

Change-Id: Ic933ef7b45955eabfa5b06e88cd195077ff9fc86

.github/workflows/android.yml

diff --git a/.github/workflows/android.yml b/.github/workflows/android.yml
index becd73f..d90a9bf 100644
--- a/.github/workflows/android.yml
+++ b/.github/workflows/android.yml
@@ -94,7 +94,7 @@
       - name: Setup Environment
         if: matrix.build_type == 'Release'
         run: |
-          echo 'CXXFLAGS=-flto' >> $GITHUB_ENV
+          echo 'CXXFLAGS=-flto=auto' >> $GITHUB_ENV
 
       - name: Configure
         run: |

.github/workflows/linux.yml

diff --git a/.github/workflows/linux.yml b/.github/workflows/linux.yml
index 24f274f..fd8fd98 100644
--- a/.github/workflows/linux.yml
+++ b/.github/workflows/linux.yml
@@ -111,7 +111,7 @@
       - name: Setup Build Environment
         if: matrix.build_type == 'Release'
         run: |
-          echo 'CXXFLAGS=-flto' >> $GITHUB_ENV
+          echo 'CXXFLAGS=-flto=auto' >> $GITHUB_ENV
 
       - name: Setup Runtime Environment
         run: |

.github/workflows/macos.yml

diff --git a/.github/workflows/macos.yml b/.github/workflows/macos.yml
index ee078e5..a71c390 100644
--- a/.github/workflows/macos.yml
+++ b/.github/workflows/macos.yml
@@ -63,7 +63,7 @@
       - name: Setup Environment
         if: matrix.build_type == 'Release'
         run: |
-          echo 'CXXFLAGS=-flto' >> $GITHUB_ENV
+          echo 'CXXFLAGS=-flto=auto' >> $GITHUB_ENV
 
       - name: Configure (iOS)
         if: matrix.target == 'ios'

.github/workflows/windows.yml

diff --git a/.github/workflows/windows.yml b/.github/workflows/windows.yml
index 605d4bc..5200d0a 100644
--- a/.github/workflows/windows.yml
+++ b/.github/workflows/windows.yml
@@ -42,8 +42,8 @@
     - name: Setup Environment
       if: ${{matrix.build_type == 'Release'}}
       run: |
-        echo 'CFLAGS=-flto' >> ~/.bash_profile
-        echo 'CXXFLAGS=-flto' >> ~/.bash_profile
+        echo 'CFLAGS=-flto=auto' >> ~/.bash_profile
+        echo 'CXXFLAGS=-flto=auto' >> ~/.bash_profile
 
     - name: Cache Build
       id: cache-build