configure: absolute path of bash

This can be used to set shebang lines during install.

configure

@@ -53,6 +53,7 @@ fi
 
 # Set several defaults (optionally specified by the user in
 # environment variables)
+BASH=${BASH:-bash}
 CC=${CC:-cc}
 CXX=${CXX:-c++}
 CFLAGS=${CFLAGS:--g -O2}
@@ -557,6 +558,16 @@ else
     errors=$((errors + 1))
 fi
 
+printf "Checking for bash... "
+if command -v ${BASH} > /dev/null; then
+    have_bash=1
+    bash_absolute=$(command -v ${BASH})
+    printf "Yes (%s).\n" "$bash_absolute"
+else
+    have_bash=0
+    printf "No. (%s not found)\n" "${BASH}"
+fi
+
 printf "Checking for python... "
 have_python=0
 
@@ -1081,6 +1092,10 @@ emacslispdir=${EMACSLISPDIR}
 # be installed
 emacsetcdir=${EMACSETCDIR}
 
+# Whether bash exists, and if so where
+HAVE_BASH = ${have_bash}
+BASH_ABSOLUTE = ${bash_absolute}
+
 # Whether there's an emacs binary available for byte-compiling
 HAVE_EMACS = ${have_emacs}
 
@@ -1262,6 +1277,10 @@ NOTMUCH_DEFAULT_XAPIAN_BACKEND=${default_xapian_backend}
 # do we have man pages?
 NOTMUCH_HAVE_MAN=$((have_sphinx))
 
+# Whether bash exists, and if so where
+NOTMUCH_HAVE_BASH=${have_bash}
+NOTMUCH_BASH_ABSOLUTE=${bash_absolute}
+
 # Name of python interpreter
 NOTMUCH_PYTHON=${python}