Fix fatal error: math.h no such file or directory

This fixes ponylang#3016 by adding lines back introduced in PR ponylang#824. This code
conditionally added llvm include directory only if it wasn't already
included. These lines were a small part of PR ponylang#2976 which adds LLVM
7.0.1 compatibility.

Makefile-ponyc

@@ -433,7 +433,17 @@ endif
 # (3) a list of include directories for a set of libraries
 # (4) a list of the libraries to link against
 llvm.ldflags := -L$(CROSS_SYSROOT)$(subst -L,,$(shell $(LLVM_CONFIG) --ldflags $(LLVM_LINK_STATIC)))
-llvm.include := $(CROSS_SYSROOT)$(shell sh -c "$(LLVM_CONFIG) --cflags $(LLVM_LINK_STATIC) | grep -oE -- '(^| )-I\s*\S+' | sed 's/^\s*-I\s*/-isystem /g'")
+llvm.include.dir := $(CROSS_SYSROOT)$(shell $(LLVM_CONFIG) --includedir $(LLVM_LINK_STATIC))
+include.paths := $(shell echo | $(CC) -v -E - 2>&1)
+ifeq (,$(findstring $(llvm.include.dir),$(include.paths)))
+# LLVM include directory is not in the existing paths;
+# put it at the top of the system list
+llvm.include := -isystem $(llvm.include.dir)
+else
+# LLVM include directory is already on the existing paths;
+# do nothing
+llvm.include :=
+endif
 llvm.libs    := $(shell $(LLVM_CONFIG) --libs $(LLVM_LINK_STATIC)) -lz -lncurses
 
 ifeq ($(OSTYPE), bsd)