qb/config.libs.sh: Extend default INCLUDES with GCC environment varia…

…bles.

C_INCLUDE_PATH and CPLUS_INCLUDE_PATH are consulted by GCC, so should
be considered by check_libs as well.

* qb/config.libs.sh (INCLUDES): Add C_INCLUDE_PATH and
CPLUS_INCLUDE_PATH values to it.

qb/config.libs.sh

@@ -32,7 +32,10 @@ CLIB=-lc
 PTHREADLIB=-lpthread
 SOCKETLIB=-lc
 SOCKETHEADER=
-INCLUDES='usr/include usr/local/include'
+TR='tr'
+INCLUDES="$(echo $C_INCLUDE_PATH | $TR ':' ' ') \
+$(echo $CPLUS_INCLUDE_PATH | $TR ':' ' ') \
+usr/include usr/local/include"
 SORT='sort'
 EXTRA_GL_LIBS=''
 VC_PREFIX=''