Tests/FortranC: Handle empty CMAKE_OSX_SYSROOT

The compiler inside Xcode requires an explicit `-isysroot`.
Replace it with the `/usr/bin/cc` wrapper.

Tests/FortranC/Flags.cmake.in

@@ -5,6 +5,9 @@ set(bld "@CMAKE_CURRENT_BINARY_DIR@/FortranC/Flags")
 # flags, remove them, and invoke the real compiler.
 set(ID "CC")
 set(COMMAND "@CMAKE_C_COMPILER@")
+if("x${COMMAND}" MATCHES "xctoolchain(/usr/bin/cc)$")
+  set(COMMAND "${CMAKE_MATCH_1}")
+endif()
 configure_file("${src}/test_opt.sh.in" "${bld}/cc.sh" @ONLY)
 set(ID "FC")
 set(COMMAND "@CMAKE_Fortran_COMPILER@")