Do not assume FHS in scripts

On systems that do not follow the Filesystem Hierarchy Standard, such as
guix, the hardcoded `/bin/pwd` will fail to be found so that the script
will fail.

Use `pwd`, instead, so that the command can be found through the normal
path search mechanism.

See https://github.com/qt/qtbase/commit/3388de698bfb9bbc456c08f03e83bf3e749df35c.

diff --git a/qtbase/configure b/qtbase/configure
index 08b49a8d..faea5b55 100755
--- a/qtbase/configure
+++ b/qtbase/configure
@@ -36,9 +36,9 @@
 relconf=`basename $0`
 # the directory of this script is the "source tree"
 relpath=`dirname $0`
-relpath=`(cd "$relpath"; /bin/pwd)`
+relpath=`(cd "$relpath"; pwd)`
 # the current directory is the "build tree" or "object tree"
-outpath=`/bin/pwd`
+outpath=`pwd`

 WHICH="which"