Suppress -Wfloat-equal warning in implementation of gsl::narrow (#1043)

In the implementation of gsl::narrow, there is a comparison `static_cast<U>(t) != u` which may be comparing two floats.
The comparison here is done purposefully to categorize ill effects of narrowing conversion, since the values being compared *should* be the same when compared with `operator==`. 
Note, using #pragma GCC will suppress this warning for both GCC and Clang.

tests/CMakeLists.txt

@@ -89,6 +89,7 @@ if(MSVC) # MSVC or simulating MSVC
         >
         $<$<CXX_COMPILER_ID:Clang>:
           -Weverything
+          -Wfloat-equal
           -Wno-c++98-compat
           -Wno-c++98-compat-pedantic
           -Wno-covered-switch-default # GTest
@@ -122,6 +123,7 @@ else()
         -Wpedantic
         -Wshadow
         -Wsign-conversion
+        -Wfloat-equal
         -Wno-deprecated-declarations # Allow tests for [[deprecated]] elements
         $<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>:
           -Weverything
@@ -227,6 +229,7 @@ if(MSVC) # MSVC or simulating MSVC
         >
         $<$<CXX_COMPILER_ID:Clang>:
           -Weverything
+          -Wfloat-equal
           -Wno-c++98-compat
           -Wno-c++98-compat-pedantic
           -Wno-missing-prototypes
@@ -250,6 +253,7 @@ else()
         -Wpedantic
         -Wshadow
         -Wsign-conversion
+        -Wfloat-equal
         $<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>:
           -Weverything
           -Wno-c++98-compat

tests/utils_tests.cpp

@@ -149,5 +149,7 @@ TEST(utils_tests, narrow)
     EXPECT_TRUE(
         narrow<std::complex<float>>(std::complex<double>(4, 2)) == std::complex<float>(4, 2));
     EXPECT_THROW(narrow<std::complex<float>>(std::complex<double>(4.2)), narrowing_error);
+
+    EXPECT_TRUE(narrow<int>(float(1)) == 1);
 }
 #endif // GSL_KERNEL_MODE