configure: Allow user to specify compiler to be used.

The environment variables CC and CXX can be set at configure time to
specify what compiler to use. This compiler will be used for any
configure-time compilation, and will also be recorded in
Makefile.config to be used during the actual build.

The compiler to be used can still be overridden at build time by using
a make variable such as:

	make CC=gcc

configure

@@ -1,5 +1,8 @@
 #! /bin/sh
 
+CC=${CC:-gcc}
+CXX=${CXX:-g++}
+
 cat <<EOF
 Welcome to Notmuch, a system for indexing, searching and tagging your email.
 
@@ -139,7 +142,7 @@ EOF
 fi
 
 printf "Checking for getline... "
-if gcc -o config/have_getline config/have_getline.c > /dev/null 2>&1
+if ${CC} -o config/have_getline config/have_getline.c > /dev/null 2>&1
 then
     printf "Yes.\n"
     have_getline=1
@@ -168,6 +171,12 @@ cat > Makefile.config <<EOF
 # changes, (and this could happen by simply calling "make" if the
 # configure script is updated).
 
+# The C compiler to use
+CC = ${CC}
+
+# The C++ compiler to use
+CXX = ${CXX}
+
 # The prefix to which notmuch should be installed
 prefix = /usr/local