CI tests: don't enable -Wextra-semi for GCC in C mode

tests/CMakeLists.txt

@@ -51,7 +51,11 @@ if (CMAKE_CXX_COMPILER_ID MATCHES "MSVC")
     set(c_flags ${c_flags} /W4 /WX /D_CRT_SECURE_NO_WARNINGS)
     set(cxx_flags ${cxx_flags} /W4 /WX /EHsc /D_CRT_SECURE_NO_WARNINGS)
 else()
-    set(c_flags ${c_flags} -Wall -Wextra -Werror -Wsign-conversion -Wstrict-prototypes -Wextra-semi)
+    set(c_flags ${c_flags} -Wall -Wextra -Werror -Wsign-conversion -Wstrict-prototypes)
+    # GCC complains about -Wextra-semi in the C compiler, only accepts it for C++
+    if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+        set(c_flags ${c_flags} -Wextra-semi)
+    endif()
     set(cxx_flags ${cxx_flags} -Wall -Wextra -Werror -Wsign-conversion -fno-rtti -fno-exceptions -Wextra-semi)
     if (CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
         set(c_flags ${c_flags} -Wno-missing-field-initializers)