FONTCONFIG_CFLAGS missing from spec.gmk.in

Filed on behalf of Ziyi Luo (luoziyi@amazon.com).

When building OpenJDK 10+33 on Amazon Linux 2012, variable `FONTCONFIG_CFLAGS` will be set during the configuration.

But the variable is not exported to `spec.gmk` because of a missing declaration in `make/autoconf/spec.gmk.in`. This results in the missing of `FONTCONFIG_CFLAGS` variable during the make. Hence fail the AWT build with the following message:

```
/home/luoziyi/build-src/src/java.desktop/unix/native/common/awt/fontpath.c:581:35: fatal error: fontconfig/fontconfig.h: No such file or directory

#include <fontconfig/fontconfig.h>
```

This is related to the previous commit: http://hg.openjdk.java.net/jdk/jdk/rev/5a270d2dfa5d#l5.7

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM:

* Make sure you do not have Fontconfig installed on your Linux machine.
* Use `--with-fontconfig="$FONTCONFIG_PATH` and start the configuration.
* Search for `FONTCONFIG_CFLAGS` in `build/linux-x86_64-normal-server-release/spec.gmk`

EXPECTED VERSUS ACTUAL BEHAVIOR:

EXPECTED -

Find `FONTCONFIG_CFLAGS:=-I/path/to/fontconfig/include` in `build/linux-x86_64-normal-server-release/spec.gmk` just like `CUPS_CFLAGS`

ACTUAL -

Cannot find `FONTCONFIG_CFLAGS`

REPRODUCIBILITY:

This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND:

Add the declaration of `FONTCONFIG_CFLAGS` in `make/autoconf/spec.gmk.in`:

--- ./make/autoconf/spec.gmk.in 2017-11-28 15:10:25.000000000 +0000
+++ /home/luoziyi/workplace/OpenJDK10Src/make/autoconf/spec.gmk.in 2017-11-30 20:18:56.497833196 +0000
@@ -294,6 +294,7 @@
FREETYPE_BUNDLE_LIB_PATH=@FREETYPE_BUNDLE_LIB_PATH@
FREETYPE_LICENSE=@FREETYPE_LICENSE@
CUPS_CFLAGS:=@CUPS_CFLAGS@
+FONTCONFIG_CFLAGS:=@FONTCONFIG_CFLAGS@
ALSA_LIBS:=@ALSA_LIBS@
ALSA_CFLAGS:=@ALSA_CFLAGS@
LIBFFI_LIBS:=@LIBFFI_LIBS@