travis: don't export RTT_CC in before_script if it's been set

This gives ways for bsp to set it's own RTT_CC.

.travis.yml

@@ -12,7 +12,7 @@ before_script:
   - "[ $RTT_TOOL_CHAIN = 'sourcery-ppc' ] && curl -s https://sourcery.mentor.com/public/gnu_toolchain/powerpc-eabi/freescale-2011.03-39-powerpc-eabi-i686-pc-linux-gnu.tar.bz2 | sudo tar xjf - -C /opt && export RTT_EXEC_PATH=/opt/freescale-2011.03/bin && /opt/freescale-2011.03/bin/powerpc-eabi-gcc --version || true"
   - "[ $RTT_TOOL_CHAIN = 'atmel-avr32' ] && curl -s http://www.atmel.com/images/avr32-gnu-toolchain-3.4.1.348-linux.any.x86.tar.gz | sudo tar xzf - -C /opt && export RTT_EXEC_PATH=/opt/avr32-gnu-toolchain-linux_x86/bin && /opt/avr32-gnu-toolchain-linux_x86/bin/avr32-gcc --version && curl -sO http://www.atmel.com/images/avr-headers-3.2.3.970.zip && unzip -qq avr-headers-3.2.3.970.zip -d bsp/$RTT_BSP || true"
   - export RTT_ROOT=`pwd`
-  - export RTT_CC='gcc'
+  - "[ x$RTT_CC == x ] && export RTT_CC='gcc' || true"
   - git clone --depth 1 https://github.com/RT-Thread/RTGUI.git $HOME/RTGUI
   - export RTT_RTGUI=$HOME/RTGUI/components/rtgui
 
@@ -20,6 +20,7 @@ script:
   - scons -C bsp/$RTT_BSP --verbose
 
 env:
+  - RTT_BSP='simulator' RTT_CC='clang-analyze'
   - RTT_BSP='at91sam9260' RTT_TOOL_CHAIN='sourcery-arm' 
   - RTT_BSP='avr32uc3b0' RTT_TOOL_CHAIN='atmel-avr32'
 #  - RTT_BSP='bf533' # no scons
@@ -59,4 +60,3 @@ env:
 #  - RTT_BSP='wh44b0' # no scons
 #  - RTT_BSP='x86' # x86
 #  - RTT_BSP='xplorer4330' # no scons
-  - RTT_BSP='simulator' RTT_CC='clang-analyze'