travis: only enable asan workaround for clang (breaks gcc)

fixes #2437

travis/run-tests.sh

@@ -7,6 +7,8 @@ cd testcases
 
 # TODO: remove this workaround once https://bugs.debian.org/836723 is fixed
 # Found at https://llvm.org/bugs/show_bug.cgi?id=27310#c8:
+if [ "$CC" = "clang" ]
+then
 	cat >fixasan.c <<EOT
 void __isoc99_printf() {}
 void __isoc99_sprintf() {}
@@ -20,6 +22,7 @@ void __cxa_throw() {} // NEW
 EOT
 	gcc fixasan.c -o fixasan.so -fPIC -shared -nostdlib
 	export LD_PRELOAD=$PWD/fixasan.so
+fi
 
 # Try running the tests in parallel so that the common case (tests pass) is
 # quick, but fall back to running them in sequence to make debugging easier.