steps-guix/argp-standalone: fix out-of-tree build include path for testsuite

argp-standalone pass1 builds in a separate build directory. Its testsuite
compiles sources that include <argp.h>, but without an explicit include path
the header in the source root is not found and build fails.

Set:
- CPPFLAGS=-I/Users/luoyanpan/CLionProjects/guix/live-bootstrap/..

in src_configure so testsuite objects can resolve argp.h during the normal
 phase.

steps-guix/argp-standalone-1.4.1/pass1.sh

@@ -13,6 +13,9 @@ src_configure() {
     mkdir build
     cd build
 
+    # argp-standalone's testsuite expects argp.h from source root even when
+    # building out-of-tree.
+    CPPFLAGS="-I${PWD}/.." \
     CC=gcc \
     AR=ar \
     RANLIB=ranlib \