configure: do not include absolute paths in -I and -L paths

On msys2, paths such as -L/e/path/to/qemu are not recognized by
the linker.  Fortunately we do not need absolute paths at all in a
non-recursive build system.

Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -944,7 +944,7 @@ Linux)
   linux="yes"
   linux_user="yes"
   kvm="yes"
-  QEMU_INCLUDES="-isystem ${source_path}/linux-headers -I$PWD/linux-headers $QEMU_INCLUDES"
+  QEMU_INCLUDES="-isystem ${source_path}/linux-headers -Ilinux-headers $QEMU_INCLUDES"
   libudev="yes"
 ;;
 esac
@@ -4266,7 +4266,7 @@ EOF
               symlink "$source_path/dtc/Makefile" "dtc/Makefile"
           fi
           fdt_cflags="-I${source_path}/dtc/libfdt"
-          fdt_ldflags="-L$PWD/dtc/libfdt"
+          fdt_ldflags="-Ldtc/libfdt"
           fdt_libs="$fdt_libs"
       elif test "$fdt" = "yes" ; then
           # Not a git build & no libfdt found, prompt for system install
@@ -5257,7 +5257,7 @@ case "$capstone" in
     else
       LIBCAPSTONE=libcapstone.a
     fi
-    capstone_libs="-L$PWD/capstone -lcapstone"
+    capstone_libs="-Lcapstone -lcapstone"
     capstone_cflags="-I${source_path}/capstone/include"
     ;;
 
@@ -6257,8 +6257,8 @@ case "$slirp" in
       git_submodules="${git_submodules} slirp"
     fi
     mkdir -p slirp
-    slirp_cflags="-I${source_path}/slirp/src -I$PWD/slirp/src"
-    slirp_libs="-L$PWD/slirp -lslirp"
+    slirp_cflags="-I${source_path}/slirp/src -Islirp/src"
+    slirp_libs="-Lslirp -lslirp"
     if test "$mingw32" = "yes" ; then
       slirp_libs="$slirp_libs -lws2_32 -liphlpapi"
     fi