Unbreak doxygen in astro/stellarium

The second CONFIGURE_ARGS overwrites the first. Issue spotted by tb,
full report here:

http://build-failures.rhaalovely.net/aarch64/2021-01-24/astro/stellarium.log

Index: Makefile
===================================================================
RCS file: /cvs/ports/astro/stellarium/Makefile,v
retrieving revision 1.69
diff -u -p -u -p -r1.69 Makefile
--- Makefile 29 Dec 2020 12:09:35 -0000 1.69
+++ Makefile 28 Jan 2021 20:55:04 -0000
@@ -5,6 +5,7 @@ COMMENT= free open source planetarium
GH_TAGNAME= v0.20.4
GH_ACCOUNT= Stellarium
GH_PROJECT= stellarium
+REVISION= 0

CATEGORIES= astro x11

@@ -36,10 +37,10 @@ CONFIGURE_ENV= HOME=${WRKDIR}
CONFIGURE_ARGS= -Wno-dev \
-DCMAKE_C_FLAGS="${CFLAGS} -I${X11BASE}/include" \
-DCMAKE_CXX_FLAGS="${CXXFLAGS} -I${X11BASE}/include" \
- -DCMAKE_DISABLE_FIND_PACKAGE_Doxygen:Bool=Yes
+ -DCMAKE_DISABLE_FIND_PACKAGE_Doxygen=ON

# XXX needs FindQt5Positioning.cmake
-CONFIGURE_ARGS= -DENABLE_GPS=0 -DENABLE_LIBGPS=0
+CONFIGURE_ARGS+= -DENABLE_GPS=0 -DENABLE_LIBGPS=0

PORTHOME= ${WRKDIR}