Cpu: disable IFUNC on GCC 4.8 if --coverage is used.

The offending flag is -fprofile-arcs and it's failing only on Ubuntu
20.04 with GCC 4.8, not with GCC 5 and not with GCC 4.8 on Arch. I
couldn't find any even vaguely related bugreport, so I suspect this has
something to with binutils or something else on that system. But since
GCC 5 works there, I don't see that much of a problem.

Unfortunately I also couldn't find any way to detect if such flag is
present, the predefined variables are exactly the same in both cases.
Which means this won't be able to detect if Corrade is built without
--coverage and then used in code that has --coverage enabled. But as I'm
probably the only person using --converage on GCC 4.8 (on the CI), I
don't think that's a problem either.

Author: mosra

CMakeLists.txt

@@ -206,10 +206,29 @@ extern \"C\" int(*fooDispatcher())() {
 int foo() __attribute__((ifunc(\"fooDispatcher\")));
 int main() { return foo() - 42; }\
         " _CORRADE_CPU_CAN_USE_IFUNC)
+    # In cases where ifunc is known to be broken, disable it by default --
+    # users can still force it to be enabled, but the initial state should not
+    # cause crashes or other strange behavior.
+    if(_CORRADE_CPU_CAN_USE_IFUNC)
+        set(_CORRADE_CPU_USE_IFUNC_DEFAULT ON)
+        # On GCC 4.8, if --coverage or -fprofile-arcs is enabled, the ifunc
+        # dispatchers cause a segfault. On Ubuntu 20.04 at least. Not the case
+        # with GCC 5 there, not the case with GCC 4.8 on Arch. Can't find any
+        # upstream bug report or commit that would be related to this.
+        if(CMAKE_CXX_COMPILER_ID AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "4.9" AND CMAKE_CXX_FLAGS MATCHES "(--coverage|-fprofile-arcs)")
+            if(NOT DEFINED CORRADE_CPU_USE_IFUNC)
+                message(WARNING "Disabling CORRADE_CPU_USE_IFUNC by default as it may crash when used together with --coverage on GCC 4.8.")
+            endif()
+            set(_CORRADE_CPU_USE_IFUNC_DEFAULT OFF)
+        endif()
+    else()
+        set(_CORRADE_CPU_USE_IFUNC_DEFAULT OFF)
+    endif()
 else()
     set(_CORRADE_CPU_CAN_USE_IFUNC OFF)
+    set(_CORRADE_CPU_USE_IFUNC_DEFAULT OFF)
 endif()
-cmake_dependent_option(CORRADE_CPU_USE_IFUNC "Allow using GNU IFUNC for runtime CPU dispatch" ON "_CORRADE_CPU_CAN_USE_IFUNC" OFF)
+cmake_dependent_option(CORRADE_CPU_USE_IFUNC "Allow using GNU IFUNC for runtime CPU dispatch" ${_CORRADE_CPU_USE_IFUNC_DEFAULT} "_CORRADE_CPU_CAN_USE_IFUNC" OFF)
 
 # Backwards compatibility for unprefixed CMake options. If the user isn't
 # explicitly using prefixed options in the first run already, accept the

doc/building-corrade.dox

@@ -557,7 +557,8 @@ Platform-specific options:
     [GNU IFUNC](https://sourceware.org/glibc/wiki/GNU_IFUNC) to be used for
     runtime dispatch in the @ref Cpu library. Available only on platforms that
     support it, which is currently only Linux with glibc and Android with API
-    30+, and there enabled by default. See
+    30+, and there enabled by default unless a problematic case is detected
+    that may cause it to misbehave. See
     @ref Cpu-usage-automatic-cached-dispatch for details and information about
     performance tradeoffs.
 -   `CORRADE_UTILITY_USE_ANSI_COLORS` --- if building for Windows, this will