https://bugs.gentoo.org/834692
https://github.com/buddhi1980/mandelbulber2/pull/972
https://github.com/microsoft/GSL/issues/466

By injecting -I/usr/include/gsl, `#include <algorithm>` starts to mean
"use ms-gsl's algorithm" rather than the C++ stdlib one. But all of the
includes for ms-gsl here are properly namespaced, e.g. <gsl/foo.h>, so
we can drop the explicit -I*.
--- a/makefiles/common.pri
+++ b/makefiles/common.pri
@@ -122,7 +122,6 @@ macx:DEFINES += "SHARED_DIR_IS_APP_DIR"
 # test hardcoded lib path for gsl in travis container 
 #QMAKE_CXXFLAGS += -I/usr/include/gsl
 
-QMAKE_CXXFLAGS += -I/usr/include/gsl
 m1:QMAKE_CXXFLAGS += -I/opt/homebrew/include
 
 # library linking