Depend on readline, remove hardcoded -ltermcap (#4042)

* depend on readline, remove hardcoded -ltermcap

Bowtie should use Spack's readline and not explicitly depend on the
system termcap (which, on CentOS, leads to linking against the
system's tinfo library).

* Add depends_on('zlib')

* Add conflict with gcc@6:

Build seems to have trouble with 6's migration to -std=gnu++14.

var/spack/repos/builtin/packages/bowtie2/bowtie2-2.3.1.patch

@@ -3,7 +3,8 @@
 @@ -26,10 +26,10 @@
 
  INC =
- LIBS = -lreadline -ltermcap -lz
+-LIBS = -lreadline -ltermcap -lz
++LIBS = -lreadline -lz
 -GCC_PREFIX = $(shell dirname `which gcc`)
 +GCC_PREFIX =
  GCC_SUFFIX =

var/spack/repos/builtin/packages/bowtie2/package.py

@@ -37,10 +37,15 @@ class Bowtie2(Package):
     version('2.2.5', '51fa97a862d248d7ee660efc1147c75f')
 
     depends_on('tbb', when='@2.3.1')
+    depends_on('readline')
+    depends_on('zlib')
 
     patch('bowtie2-2.2.5.patch', when='@2.2.5', level=0)
     patch('bowtie2-2.3.1.patch', when='@2.3.1', level=0)
 
+    # seems to have trouble with 6's -std=gnu++14
+    conflicts('%gcc@6:')
+
     def install(self, spec, prefix):
         make()
         mkdirp(prefix.bin)