Apple: Merge per-arch sysroot parameters if all are the same

Since commit a9b41195d2 (Handle multi-arch sysroots on Apple platforms,
2019-07-26, v3.17.0-rc1~287^2), CMake supports "fat" builds with
different sysroots. Those are passed to the compiler with the
`-Xarch_<xyz>` parameter.

Unfortunately this breaks the Compiler Cache (ccache) because
it does not support those compiler flags:

    https://github.com/ccache/ccache/blob/v3.7.9/src/ccache.c#L2700-L2705

Restore the caching ability for certain "fat" build configurations (e.g.
`arm64` and `armv7`) where the sysroot is the same for all selected
architectures and thus a plain `-isysroot` parameter could be used.

Source/cmLocalGenerator.cxx

@@ -1789,6 +1789,26 @@ std::string cmLocalGenerator::GetLinkLibsCMP0065(
   return linkFlags;
 }
 
+bool cmLocalGenerator::AllAppleArchSysrootsAreTheSame(
+  const std::vector<std::string>& archs, const char* sysroot)
+{
+  if (!sysroot) {
+    return false;
+  }
+
+  for (std::string const& arch : archs) {
+    std::string const& archSysroot = this->AppleArchSysroots[arch];
+    if (cmIsOff(archSysroot)) {
+      continue;
+    }
+    if (archSysroot != sysroot) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
 void cmLocalGenerator::AddArchitectureFlags(std::string& flags,
                                             cmGeneratorTarget const* target,
                                             const std::string& lang,
@@ -1814,7 +1834,8 @@ void cmLocalGenerator::AddArchitectureFlags(std::string& flags,
       std::string("CMAKE_") + lang + "_SYSROOT_FLAG";
     const char* sysrootFlag = this->Makefile->GetDefinition(sysrootFlagVar);
     if (sysrootFlag && *sysrootFlag) {
-      if (!this->AppleArchSysroots.empty()) {
+      if (!this->AppleArchSysroots.empty() &&
+          !this->AllAppleArchSysrootsAreTheSame(archs, sysroot)) {
         for (std::string const& arch : archs) {
           std::string const& archSysroot = this->AppleArchSysroots[arch];
           if (cmIsOff(archSysroot)) {

Source/cmLocalGenerator.h

@@ -528,6 +528,8 @@ private:
                                    int targetType);
 
   void ComputeObjectMaxPath();
+  bool AllAppleArchSysrootsAreTheSame(const std::vector<std::string>& archs,
+                                      const char* sysroot);
 };
 
 #if !defined(CMAKE_BOOTSTRAP)