--- gmic-3.6.2.old/gmic-qt/gmic_qt.pro
+++ gmic-3.6.2/gmic-qt/gmic_qt.pro
@@ -16,7 +16,7 @@
 }
 
 # Possible values are "gcc" or "clang"
-!defined(COMPILER,var) { COMPILER = gcc }
+!defined(COMPILER,var) { COMPILER = clang }
 
 # Possible values are "on" or "off"
 !defined(LTO,var) { LTO=off }
--- gmic-3.6.2.old/src/Makefile
+++ gmic-3.6.2/src/Makefile
@@ -217,8 +217,8 @@
 OPT_LIBS = # ${FLTO}
 endif
 ifdef IS_CLANG
-OPT_CFLAGS = -O3 -mtune=generic --stdlib=libc++ # '-Ofast' makes things unstable
-OPT_LIBS = ${FLTO}
+OPT_CFLAGS = -O3 -mtune=generic # '-Ofast' makes things unstable
+OPT_LIBS =
 endif
 ifdef icpc
 OPT_CFLAGS = -fast
@@ -234,7 +234,7 @@
 # Enable parallelization, using OpenMP.
 # (http://www.openmp.org/)
 ifdef IS_CLANG
-OPENMP_CFLAGS = -I/usr/lib/gcc/x86_64-linux-gnu/9/include -Dcimg_use_openmp -fopenmp=libomp
+OPENMP_CFLAGS = -Dcimg_use_openmp -fopenmp=libomp
 OPENMP_LIBS = -lomp
 else
 OPENMP_CFLAGS = -fopenmp -Dcimg_use_openmp