[math] minor update for math lib building

use PAPARAZZI_HOME and install to /usr/local by default

sw/airborne/math/Makefile

@@ -8,14 +8,15 @@ CC= gcc
 CFLAGS= -fpic
 INCLUDES= -I$(PAPARAZZI_SRC)/sw/include -I$(PAPARAZZI_SRC)/sw/airborne
 
-# build in ../../../var/build/math
-ifndef BUILDDIR
-BUILDDIR=../../../var/build/math
+ifeq ($(PAPARAZZI_HOME),)
+PAPARAZZI_HOME=$(PAPARAZZI_SRC)
 endif
 
-ifndef PREFIX
-PREFIX=/usr
-endif
+# build in $(PAPARAZZI_HOME)/var/build/math
+BUILDDIR ?= $(PAPARAZZI_HOME)/var/build/math
+
+PREFIX ?= /usr/local
+
 LIB_INSTALLDIR=${PREFIX}/lib
 INCLUDE_INSTALLDIR=${PREFIX}/include/pprz
 PKGCONFIG_INSTALLDIR=${PREFIX}/lib/pkgconfig
@@ -48,3 +49,4 @@ $(BUILDDIR)/%.o: %.c
 clean:
 	$(Q)rm -f $(BUILDDIR)/*.o $(BUILDDIR)/$(LIBNAME).so
 
+.PHONY: all clean shared_lib install_shared_lib

sw/airborne/math/README

@@ -11,11 +11,11 @@ HOWTO install a shared library to use in other projects
 2. Install library: in this folder, type
   make install_shared_lib
 
-  the default install dir is /usr
+  the default install dir is /usr/local
   and will install files in
-  /usr/lib
-  /usr/lib/pkgconfig
-  /usr/include/pprz
+  /usr/local/lib
+  /usr/local/lib/pkgconfig
+  /usr/local/include/pprz
 
   to change the install dir: PREFIX=<your_install_dir> make install_shared_lib