Use PKG_CONFIG for libicu foundation.CFLAGS (#722)

- Mirrors use for foundation.LDFLAGS, allowing libicu to be
  used if installed in non-default include directory

Author: spevans

build.py

@@ -15,7 +15,7 @@
 
 swift_cflags = ['-DDEPLOYMENT_RUNTIME_SWIFT']
 if Configuration.current.target.sdk == OSType.Linux:
-	foundation.CFLAGS = '-DDEPLOYMENT_TARGET_LINUX -D_GNU_SOURCE -DCF_CHARACTERSET_DATA_DIR="CoreFoundation/CharacterSets"'
+	foundation.CFLAGS = '`${PKG_CONFIG} icu-uc icu-i18n --cflags-only-I` -DDEPLOYMENT_TARGET_LINUX -D_GNU_SOURCE -DCF_CHARACTERSET_DATA_DIR="CoreFoundation/CharacterSets"'
 	foundation.LDFLAGS = '${SWIFT_USE_LINKER} -Wl,@./CoreFoundation/linux.ld -lswiftGlibc `${PKG_CONFIG} icu-uc icu-i18n --libs` -Wl,-Bsymbolic '
 	Configuration.current.requires_pkg_config = True
 elif Configuration.current.target.sdk == OSType.FreeBSD: