Fix compiling v2.x with C++17 + Clang 5 + libstdc++ v5

This basically tests the combination where the compiler supports most
of C++17 but the library does not.

.travis.yml

@@ -258,6 +258,15 @@ matrix:
       addons: *gcc7
       env: COMPILER='g++-7' EXAMPLES=1 COVERAGE=1 EXTRAS=1 CPP17=1
 
+    - os: linux
+      dist: xenial
+      compiler: clang
+      addons:
+          apt:
+              sources: *all_sources
+              packages: ['clang-5.0']
+      env: COMPILER='clang++-5.0' CPP17=1
+
     - os: linux
       dist: xenial
       compiler: clang

include/internal/catch_compiler_capabilities.h

@@ -234,7 +234,7 @@
   // Check if byte is available and usable
   #  if __has_include(<cstddef>) && defined(CATCH_CPP17_OR_GREATER)
   #    include <cstddef>
-  #    if __cpp_lib_byte > 0
+  #    if defined(__cpp_lib_byte) && (__cpp_lib_byte > 0)
   #      define CATCH_INTERNAL_CONFIG_CPP17_BYTE
   #    endif
   #  endif // __has_include(<cstddef>) && defined(CATCH_CPP17_OR_GREATER)

projects/SelfTest/UsageTests/Generators.tests.cpp

@@ -58,12 +58,12 @@ TEST_CASE("tables", "[generators]") {
 
 // Structured bindings make the table utility much nicer to use
 TEST_CASE( "strlen2", "[approvals][generators]" ) {
-    auto [test_input, expected] = GENERATE( table<std::string, size_t>({
-            {"one", 3},
-            {"two", 3},
-            {"three", 5},
-            {"four", 4}
-        }));
+    using tuple_type = std::tuple<std::string, int>; // see above workaround
+    auto [test_input, expected] =
+        GENERATE( table<std::string, size_t>( { tuple_type{ "one", 3 },
+                                                tuple_type{ "two", 3 },
+                                                tuple_type{ "three", 5 },
+                                                tuple_type{ "four", 4 } } ) );
 
     REQUIRE( test_input.size() == expected );
 }
@@ -99,11 +99,9 @@ TEST_CASE( "strlen3", "[generators]" ) {
 static auto eatCucumbers( int start, int eat ) -> int { return start-eat; }
 
 SCENARIO("Eating cucumbers", "[generators][approvals]") {
-
-    auto [start, eat, left] = GENERATE( table<int,int,int> ({
-            { 12, 5, 7 },
-            { 20, 5, 15 }
-        }));
+    using tuple_type = std::tuple<int, int, int>;
+    auto [start, eat, left] = GENERATE( table<int, int, int>(
+        { tuple_type{ 12, 5, 7 }, tuple_type{ 20, 5, 15 } } ) );
 
     GIVEN( "there are " << start << " cucumbers" )
     WHEN( "I eat " << eat << " cucumbers" )