The option -Werror=missing-include-dirs causes the build to fail if
non-exsistent directories are added via -I.  Removing the rest of the -Werror*
flags to ensure no new bugs get created because a -Werror flag stopped the
build.

--- hexchat-2.12.2/configure.ac.old	2016-10-15 13:40:30.585210113 -0700
+++ hexchat-2.12.2/configure.ac	2016-10-15 13:40:44.937008119 -0700
@@ -634,17 +634,10 @@ AX_APPEND_COMPILE_FLAGS([\
 	-Wno-unused-parameter \
 	-Wno-sign-compare \
 	-Wno-pointer-sign \
 	-Wno-missing-field-initializers \
 	-Wno-unused-result \
-	-Werror=format-security \
-	-Werror=init-self \
-	-Werror=declaration-after-statement \
-	-Werror=missing-include-dirs \
-	-Werror=date-time \
-	-Werror=implicit-function-declaration \
-	-Werror=pointer-arith \
 ])
 
 AS_IF([test "$stack_protector" = "yes"], [
 	AX_APPEND_COMPILE_FLAGS([ \
 		-fstack-protector-strong \