Rewrite FindQuadMath to provide imported targets

cmake/modules/FindQuadMath.cmake

-# .. cmake_module::
-#
-#    Find the GCC Quad-Precision library
-#
-#    Sets the following variables:
-#
-#    :code:`QUADMATH_FOUND`
-#       True if the Quad-Precision library was found.
-#
-#
-
-
-# search for the header quadmath.h
-include(CheckIncludeFile)
-check_include_file(quadmath.h QUADMATH_HEADER)
-
-include(CheckCSourceCompiles)
-include(CMakePushCheckState)
+#[=======================================================================[.rst:
+FindQuadMath
+------------
+
+Find the GCC Quad-Precision library
+
+This module checks if the used compiler has built-in support for QuadMath
+by compiling a small source file.
+
+Imported Targets
+^^^^^^^^^^^^^^^^
+
+This module provides the following imported targets, if found:
+
+``QuadMath::QuadMath``
+  Library to link against if QuadMath should be used.
+
+Result Variables
+^^^^^^^^^^^^^^^^
+
+This will define the following variables:
+
+``QuadMath_FOUND``
+  True if the Quad-Precision library was found.
 
-cmake_push_check_state() # Save variables
+#]=======================================================================]
+
+# Add a feature summary for this package
+include(FeatureSummary)
+set_package_properties(QuadMath PROPERTIES
+  DESCRIPTION "GCC Quad-Precision Math Library"
+  URL "https://gcc.gnu.org/onlinedocs/libquadmath"
+)
+
+# Check if QuadMath support is built into the compiler
+include(CheckCXXSourceCompiles)
+include(CMakePushCheckState)
+cmake_push_check_state()
 set(CMAKE_REQUIRED_LIBRARIES quadmath)
-check_c_source_compiles("
+check_cxx_source_compiles("
 #include <quadmath.h>
 
 int main ()
@@ -27,33 +45,31 @@ int main ()
   __float128 r = 1.0q;
   r = strtoflt128(\"1.2345678\", NULL);
   return 0;
-}" QUADMATH_COMPILES)
-cmake_pop_check_state()
+}" QuadMath_COMPILES)
+cmake_pop_check_state()  # Reset CMAKE_REQUIRED_XXX variables
 
+if(QuadMath_COMPILES)
+  # Use additional variable for better report message
+  set(QuadMath_VAR "(Supported by compiler)")
+endif()
 
+# Report that package was found
 include(FindPackageHandleStandardArgs)
-find_package_handle_standard_args(
-  "QuadMath"
+find_package_handle_standard_args(QuadMath
   DEFAULT_MSG
-  QUADMATH_HEADER
-  QUADMATH_COMPILES
+  QuadMath_VAR QuadMath_HEADER QuadMath_COMPILES
 )
 
-# text for feature summary
-set_package_properties("QuadMath" PROPERTIES
-  DESCRIPTION "GCC Quad-Precision library")
-
-# set HAVE_QUADMATH for config.h
-set(HAVE_QUADMATH ${QUADMATH_FOUND})
-
-# -fext-numeric-literals is a GCC extension not available in other compilers like clang
-if(${CMAKE_CXX_COMPILER_ID} STREQUAL GNU)
-  set(_QUADMATH_EXT_NUMERIC_LITERALS "-fext-numeric-literals")
-endif()
+# add imported target for quadmath
+if(QuadMath_FOUND AND NOT TARGET QuadMath::QuadMath)
+  # Compiler supports QuadMath: Add appropriate linker flag
+  add_library(QuadMath::QuadMath INTERFACE IMPORTED)
+  target_link_libraries(QuadMath::QuadMath INTERFACE quadmath)
 
-# register all QuadMath related flags
-if(HAVE_QUADMATH)
-  dune_register_package_flags(COMPILE_DEFINITIONS "ENABLE_QUADMATH=1" "_GLIBCXX_USE_FLOAT128=1"
-                              COMPILE_OPTIONS ${_QUADMATH_EXT_NUMERIC_LITERALS}
-                              LIBRARIES "quadmath")
+  target_compile_definitions(QuadMath::QuadMath INTERFACE
+    _GLIBCXX_USE_FLOAT128
+  )
+  target_compile_options(QuadMath::QuadMath INTERFACE
+    $<$<CXX_COMPILER_ID:GNU>:-fext-numeric-literals>
+  )
 endif()