Build fixes for updating dEQP's version of amber (#634)

* CMakeLists.txt: Remove clang-only flags from gcc builds

Fixes: #628

* Avoid implicit conversion of size_t to double

Fixes:  #628
diff --git a/CMakeLists.txt b/CMakeLists.txt
index aa09507..80a249f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -194,18 +194,20 @@
       -Wall
       -Werror
       -Wextra
-      -Wno-c++98-compat
-      -Wno-c++98-compat-pedantic
-      -Wno-format-pedantic
       -Wno-padded
       -Wno-switch-enum
       -Wno-unknown-pragmas
-      -Wno-unknown-warning-option
       -pedantic-errors
     )
 
     if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
-      target_compile_options(${TARGET} PRIVATE -Weverything)
+      target_compile_options(${TARGET} PRIVATE
+        -Wno-c++98-compat
+        -Wno-c++98-compat-pedantic
+        -Wno-format-pedantic
+        -Wno-unknown-warning-option
+        -Weverything
+      )
     endif()
   endif()
 
diff --git a/src/buffer.cc b/src/buffer.cc
index f160923..b349832 100644
--- a/src/buffer.cc
+++ b/src/buffer.cc
@@ -193,7 +193,7 @@
   for (const auto val : diffs)
     sum += (val * val);
 
-  sum /= diffs.size();
+  sum /= static_cast<double>(diffs.size());
   double rmse = std::sqrt(sum);
   if (rmse > static_cast<double>(tolerance)) {
     return Result("Root Mean Square Error of " + std::to_string(rmse) +