fix a abseil build failure on Windows in GitHub

Author: duncanpo

.github/workflows/build_and_test.yml

@@ -56,7 +56,7 @@ jobs:
               shell: cmd
               run: |
                   call "C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Auxiliary\Build\vcvarsall.bat" x64
-                  cmake -S . -B build -G Ninja -DCMAKE_CXX_COMPILER="cl.exe" -DCMAKE_C_COMPILER="cl.exe" -DCMAKE_BUILD_TYPE=Release -DWITH_EXAMPLES=ON -DWITH_OTLP_GRPC=ON -DUSE_BATCH_FOR_MCC=ON -DCMAKE_INSTALL_PREFIX=${{ env.OPENTELEMETRY_MATLAB_INSTALL }}
+                  cmake -S . -B build -G Ninja -DCMAKE_CXX_COMPILER="cl.exe" -DCMAKE_C_COMPILER="cl.exe" -DCMAKE_BUILD_TYPE=Release -DWITH_EXAMPLES=ON -DWITH_OTLP_GRPC=ON -DUSE_BATCH_FOR_MCC=ON -DFETCH_VCPKG -DCMAKE_INSTALL_PREFIX=${{ env.OPENTELEMETRY_MATLAB_INSTALL }}
                   cmake --build build --config Release --target install
             - name: Run tests
               env:

.github/workflows/publish_mltbx.yml

@@ -54,7 +54,7 @@ jobs:
               shell: cmd
               run: |
                   call "C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Auxiliary\Build\vcvarsall.bat" x64
-                  cmake -S . -B build -G Ninja -DCMAKE_CXX_COMPILER="cl.exe" -DCMAKE_C_COMPILER="cl.exe" -DCMAKE_BUILD_TYPE=Release -DWITH_OTLP_GRPC=ON -DCMAKE_INSTALL_PREFIX=${{ env.OPENTELEMETRY_MATLAB_INSTALL }}
+                  cmake -S . -B build -G Ninja -DCMAKE_CXX_COMPILER="cl.exe" -DCMAKE_C_COMPILER="cl.exe" -DCMAKE_BUILD_TYPE=Release -DWITH_OTLP_GRPC=ON -DFETCH_VCPKG -DCMAKE_INSTALL_PREFIX=${{ env.OPENTELEMETRY_MATLAB_INSTALL }}
                   cmake --build build --config Release --target install
             - name: Compress into single artifact
               working-directory: ${{ github.workspace }}

CMakeLists.txt

@@ -10,15 +10,16 @@ cmake_policy(SET CMP0074 NEW)
 include(FetchContent)
 
 # check if VCPKG_ROOT is defined, which should point to an existing installation
-if(DEFINED ENV{VCPKG_ROOT})
+# FETCH_VCPKG is a switch that can be defined to ignore VCPKG_ROOT
+if(DEFINED ENV{VCPKG_ROOT} AND NOT DEFINED FETCH_VCPKG)
    # Autodetect vcpkg toolchain
    if(NOT DEFINED CMAKE_TOOLCHAIN_FILE)
        string(REPLACE "\\" "/" CMAKE_TOOLCHAIN_FILE "$ENV{VCPKG_ROOT}/scripts/buildsystems/vcpkg.cmake")
    endif()
 else()
    # VCPKG_ROOT not defined, get it using FetchContent
 
-   if(DEFINED VCPKG_PREFIX})
+   if(DEFINED VCPKG_PREFIX)
        # download location specified
        string(REPLACE "\\" "/" VCPKG_PREFIX "${VCPKG_PREFIX}")
    else()